Semiconductor thin film, method of producing the same, apparatus for producing the same, semiconductor device and method of producing the same

ABSTRACT

In a polycrystalline silicon thin film transistor, a semiconductor device having a high field effect mobility is achieved by increasing a grain size of a silicon thin film. First, an insulation layer having a two-layer structure is formed on a transparent insulated substrate  201 . In the insulation layer, a lower insulation layer  202 , which is in contact with the transparent insulating substrate  201 , is made to have a higher thermal conductivity than an upper insulation layer  203 . Thereafter, the upper insulation layer  203  is patterned so that a plurality of stripes are formed thereon. Subsequently, an amorphous silicon thin film  204  is formed on the patterned insulation layer, and the insulation layer is irradiated with a laser light scanning in a direction parallel to the stripe pattern on the upper insulation layer  203 . Thus, the amorphous silicon thin film  203  is formed into a polycrystalline silicon thin film  210.

TECHNICAL FIELD

The present invention relates to a semiconductor film for use in thinfilm transistors (TFTs) used in liquid crystal displays, photosensorssuch as linear sensors, photovoltaic devices such as solar cells, memoryLSIs for SRAMs (static random access memories) and the like. Theinvention also relates to a method and an apparatus for producing thesemiconductor film. More particularly, the semiconductor film is acrystalline semiconductor thin film formed on, for example, a glasssubstrate or the like, by laser annealing an amorphous material or thelike. The invention further relates to semiconductor device using thesemiconductor thin film and a method of producing the device.

BACKGROUND ART

Conventionally, high-quality silicon semiconductor thin films used inthin film transistors (TFTs) or the like have been fabricated on anamorphous insulating substrates by using plasma CVD methods and plasmaCVD apparatuses utilizing glow discharge. The hydrogenated amorphoussilicon (a-Si) films produced by these manufacturing methods andapparatuses have been improved over many years of research anddevelopment and reached to a standard applicable as high qualitysemiconductor thin films. The hydrogenated amorphous thin films havefound use in electric optical devices such as switching transistors forpixels in active matrix liquid crystal displays for lap-top or note-typepersonal computers, engineering workstations, and automobile navigationsystems, photosensors for image sensors in facsimile machines, and solarbatteries for electronic calculators, and in various integratedcircuits. One of the most significant advantages of the hydrogenatedamorphous silicon is that the formation with high reproducibility andstability on a large-area substrate is achieved at a process temperatureas low as 300° C.

Meanwhile, the recent advancement of increased device sizes and greaterpixel densities (higher definitions) in displays and image sensors hasled to the demand for silicon semiconductor thin films that can achievefurther high speed driving. In addition, in order to reduce deviceweight and manufacturing cost, such thin films should be applicable todriver elements formed in the peripheral circuit area of a liquidcrystal display, which also requires the thin films to be capable ofoperating at high speed. However, the foregoing amorphous silicon showsa field effect mobility of 1.0 cm²/V·sec at best and thus cannot attainelectric characteristics that can meet the requirements as mentionedabove.

In view of the problems, research has been conducted on techniques forimproving the field effect mobility and the like by forming asemiconductor thin film having crystallinity, and the developedmanufacturing processes include:

(1) a manufacturing method in which by mixing a silane gas with hydrogenor SiF₄, and employing a plasma CVD method, the deposited,thin film iscrystallized; and

(2) a manufacturing method in which by using amorphous silicon as aprecursor, the crystallization of the film is effected.

In the method (1), the crystallization proceeds along with the formationof the semiconductor thin film, and the substrate must be heated to arelatively high temperature (600° C. or higher). This necessitates theuse of a heat-resistant substrate such as costly quartz substrates,makes it difficult to use low-priced glass substrates, and therefore hasa drawback of high manufacturing cost. Specifically, for example,Corning 7059 glass widely used in active matrix type liquid crystaldisplays has a glass transition temperature of 593° C., and therefore ifsubjected to a heating treatment at 600° C. or higher, the glasssubstrate will undergo considerable mechanical deformations such asshrinkage and strain, which makes it difficult to appropriately performthe forming processes of semiconductor circuits and the producingprocesses of liquid crystal panels. Furthermore, when amulti-dimensional integration is desired, there is a possibility ofthermally damaging the previously-formed circuit area.

In the method (2) above, an amorphous silicon thin film is formed on asubstrate, and the formed thin film is heated to obtain apolycrystalline silicon (polycrystal silicon: p-Si) thin film. Thismethod generally utilizes a solid phase epitaxy method in which the heattreatment is performed at approximately 600° C. for a long time, and alaser annealing method (especially an excimer laser annealing method).

In the solid phase epitaxy method, the substrate on which an amorphousthin film is formed needs to be heated and maintained at,a temperatureof 600° C. or higher for 20 hours or longer, and thus this method alsohas drawbacks of high manufacturing cost and so forth.

In the excimer laser annealing method, an amorphous silicon thin film isirradiated with an excimer laser light, which is a UV light having alarge light energy, to cause crystallization, as disclosed in, forexample, IEEE Electron Device Letters, 7 (1986) pp. 276-8, IEEETransactions on Electron Devices, 42 (1995) pp. 251-7. This methodthereby achieves, without directly heating the glass substrate, apolycrystalline silicon thin film having relatively good electricalcharacteristics such as a high field effect mobility (higher than 100cm²/V·sec). More specifically, amorphous silicon has a transmissivitycharacteristic as shown in FIG. 1 and, for example, shows an absorptioncoefficient of about 10⁶ cm⁻¹ for a laser beam having a wavelength of308 nm by a XeCl excimer laser. Therefore, most of the laser beam isabsorbed in the layer from the surface to a depth of about 100 Å, thesubstrate temperature is not raised significantly (to approximately nothigher than 600° C.), and amorphous silicon alone is brought to a hightemperature to cause crystallization (polycrystallization or singlecrystallization). This allows the use of low-cost glass substrates. Inaddition, it is possible to irradiate a limited area of the substratewith the light beam to crystallize, and this allows a multi-dimensionalintegration in which a pixel region not requiring high speedcharacteristics so much is left to be an amorphous thin film, while aperipheral region of the pixel region is crystallized so as to form adriver circuit thereon. Further, it is also possible to form highquality crystalline thin films in a specific region on a substrate oneafter another, without thermally damaging the circuits already formed onthe substrate. Furthermore, this technique permits the integration ofCPUs (central processing units) and the like on the same substrate.

As an example of a semiconductor device using p-Si as described above,explained below is a typical construction of a TFT and a manufacturingmethod thereof.

FIGS. 2(a) and 2(b) schematically show a TFT 110 having a coplanarstructure. FIG. 2(a) is a plan view of the TFT 110, and FIG. 2(b) is across sectional view thereof taken along the line P-P′ in FIG. 2(a). Asshown in FIGS. 2(a) and 2(b), the TFT 110 has an insulating substrate111 on which there are provided a undercoat layer 112, a p-Si film 113,a first insulating film (gate insulting film) 114, a second insulatingfilm 116, and three electrodes, namely, a gate electrode 115, a sourceelectrode 117 s, and a drain electrode 117 d. The p-Si film 113 is acrystalline semiconductor layer composed of Si (silicon). The p-Si film113 is formed on the undercoat layer 112, patterned in a predeterminedshape. The p-Si film 113 comprises a channel region 113 a, a sourceregion 113 b, and a drain region 113 c, and the source region 113 b andthe drain region 113 c are disposed on both sides of the channel region113 a. The source region 113 b and drain region 113 c are formed bydoping impurity ions such as phosphorus ions and boron ions.

The first insulating film 114 is made of, for example, silicon dioxide(SiO₂), and is formed over the p-Si film 113 and the undercoat layer112. The gate electrode 115 is a metal thin film made of, for example,aluminum (Al) or the like. The gate electrode 115 is disposed above thefirst insulating film 114 at the position corresponding to the channelregion 113 a of the p-Si film 113. The second insulating film 116 ismade of, for example, SiO₂, and stacked above the gate electrode 115 andthe first insulating film 114.

The first insulating film 114 and the second insulating film 116 eachhave a contact hole 118 formed so as to reach the source region 113 b orthe drain region 113 c in the p-Si film 113. Via the contact hole 118,the source electrode 117 s and the drain electrode 117 d are in contactwith the source region 113 b or the drain region 113 c. In a region notbeing the cross section shown in the figure, the gate electrode 115, thesource electrode 117 s, and the drain electrode 117 d are patterned in apredetermined shape to form a wiring pattern.

The TFT 110 is produced in the following manner. First, the undercoatlayer 112, made of for example SiO₂, is formed on the insulatingsubstrate 111. This prevents impurities from diffusing into a p-Si film133 to be formed later. Next, on the undercoat layer 112, an a-Si film(not shown) as the foregoing amorphous silicon is deposited by, forexample, a plasma CVD method. The a-Si film is then patterned into apredetermined shape by etching. It is noted that the patterning may beperformed after the crystallization. Thereafter, the a-Si film isirradiated with an excimer laser having a short wavelength, and cooled(laser annealing). Thereby, the a-Si film is modified, i.e., the a-Sifilm is polycrystallized to form a p-Si film 113. Note here that thea-Si film has a large light absorption coefficient in a short wavelengthrange, and therefore, by employing an excimer laser as the energy beam,it is possible to selectively heat the a-Si film alone. Accordingly,temperature rise of the insulating substrate 111 is suppressed, and thisleads to an advantage that low-cost substrates such as glass substratescan be employed for the insulating substrate 111.

On the p-Si film 113 thus formed, the first insulating film 114 isdeposited by an atmospheric pressure CVD (chemical vapor deposition)method, and the gate electrode 115 is formed on the first insulatingfilm 114. Thereafter, using the gate electrode 115 as a mask, impurityions for serving as either donors or acceptors, specifically, suchimpurity ions as phosphorus ions and boron ions, are implanted into thep-Si film 113 by using, for example, an ion doping method. Thus, on thep-Si film 113, the channel region 113 a, the source region 113 b, andthe drain region 113 c are formed.

Next, the second insulating film 116 is formed on the gate electrode115. Thereafter the contact holes 118 are formed, and aluminum, forexample, is deposited, and patterning is performed to form the sourceelectrode 117 s and the drain electrode 117 d.

The pulsed type laser such as the excimer laser used in the formingprocess of p-Si films as described above has a large output power, andby irradiating the substrate with the laser light in such a manner thatthe substrate is scanned by the laser light with a line-like shape whilethe substrate is being moved, a large-area amorphous silicon can becrystallized at one time, which is an advantage in the large-scaleproduction of semiconductor devices. However, such a laser has adrawback that it is difficult to improve quality of the formed crystals.Specifically, a laser of this type has a very short irradiation time for1 pulse; namely approximately several ten nanoseconds, and causes alarge temperature difference between a time during which the irradiationis performed and a time during which the irradiation is not performed islarge, and thereby, the fused silicon film crystallizes during theprocess of being rapidly cooled. Accordingly, the control of a degree ofcrystal growth and a crystal orientation is difficult, which leads tomany drawbacks as follows: sufficient crystal growth tends not to occur,thereby crystal grain sizes are reduced and densities of grainboundaries are increased, uneveness in crystals is increased, andfurther crystal defects tend to increase. More specifically, in theprocess of cooling after the irradiation with the laser, crystal nucleiare formed in a disorderly manner, and the disorderly-formed crystalnuclei in turn grow in disorderly directions. The crystal growth stopsin a state where crystal grains collide with each other. The crystalgrains formed through such a growth process results in small grainshaving random shapes. Consequently, in the resulting poly-Si film, alarge number of grain boundaries are present and therefore chargecarriers cannot transfer smoothly, which causes the film to have poorTFT characteristics such as field effect mobilities.

Now, the mechanism of the crystal growth and the reasons why a goodcrystal growth is difficult to attain are detailed below. The aforesaidexcimer laser is generated by exciting a mixed gas containing a rare gassuch as Xe and Kr and a halogen such as Cl and F with the use of anelectron beam. However, the laser as it is cannot be suitably used. Byusing an optical system called a beam homogenizer, the light beamgenerally used in laser annealing is shaped into a light beam having ahomogeneous light intensity and having a line-like shape or arectangular shape with each line segment being about severalcentimeters. The light beam thus shaped is employed for the technique inthe crystallization of non-single crystalline thin films (normally,amorphous thin films), the technique in which the substrate is scannedwith the light beam.

Yet, this technique has several problems to be solved, such as pooruniformity in crystal grain sizes and in crystallinity, unstabletransistor characteristics, and low field effect mobilities. In view ofthese problems, the following techniques have been suggested.

(1) By covering part of the surface to be irradiated with a reflectivefilm or absorption film so as to control light absorption on the surfaceof the thin film, a light intensity distribution is formed to guide anorientation of crystal growth. (2) A substrate is heated (at 400° C.),and the laser is applied to the heated substrate, so that a smoothcrystallization takes place. (Extended Abstracts of the 1991International Conference on Solid State Devices and Materials, Yokohama,1991, pp. 623-5)

Additionally, a technique disclosed in Jpn. J. Appl. Phys. 31 (1992) pp.4550-4 is also known.

As shown in FIG. 3, a glass substrate 121 having a precursorsemiconductor thin film 122 formed thereon is placed on a substratestage 124. With a substrate stage 124 heated at about 400° C. by asubstrate heater 125, a laser beam 123 a of an excimer laser 123 isapplied to the precursor semiconductor thin film 122. The additionalheating of the glass substrate in the irradiation with laser beamachieves high crystal quality, i.e., relatively large and uniformcrystal grains, thereby improving the electrical characteristics.

Technique (1) described above can be employed to obtain a singlecrystal, whereas technique (2) above is relatively easy to implement andcan suppress the variation of field effect mobilities within ±10%.However, these techniques have the following problems and cannot meetthe requirements in recent technical trends towards multi-dimensionalintegration and further cost reduction.

Specifically, technique (1) above requires a step of providing areflective film etc., which complicates the manufacturing process andadds the manufacturing cost. Furthermore, since providing a reflectivefilm etc. in a narrow, limited space is difficult, crystallization ofspecific regions with a very small size is accordingly problematic.

On the other hand, technique (2) involves a step of heating thesubstrate, which reduces the productivity. Although the substrate is notheated at such a temperature as required in a solid phase epitaxymethod, the step of heating and cooling the substrate takes a long time(30 minutes to 1 hour, for example), which reduces the throughput. Thisproblem becomes more serious as a substrate area increases, since alarger substrate requires a longer time for heating and cooling toalleviate the deformation of the substrate. In addition, this techniquecan reduce the variation of field effect mobilities to a certain extent,but cannot sufficiently increase the field effect mobilities per se, andtherefore it is not suitable for producing circuits in which high speedoperation is required. In order not to cause the deformation etc. of theglass substrate, the substrate cannot be heated in excess of 550° C.,and therefore it is difficult to attain further higher crystal quality.Moreover, this technique involves heating the entire substrate, andtherefore is not suitable for the crystallization in a limited region(specific region) on the substrate.

As has been described above, both techniques (1) and (2) above haveproblems such as high manufacturing cost. Particularly, techniques (1)and (2) above (including other prior art techniques) have a significantproblem of the difficulty in realizing diverse and multi-dimensionallayer stacking. In other words, the methods for controlling thetemperature distribution employed by these techniques are not suitablefor selectively forming on a single substrate both a circuit regionwhere high speed operation is required (polycrystallized region) andanother circuit region where not such high speed is required (amorphousregion). For this reason, by these techniques, it is difficult toachieve both a high degree of integration and cost reduction at the sametime.

The technique capable of crystallizing only a predetermined limitedregion is useful, and this will be elaborated upon now. Prior art laserannealing methods have employed a light beam having such acharacteristic that the side (edge) of the beam is steep, and the top isflat (the energy intensity per unit area is uniform), as shown in FIG.4. Poly-Si thin films produced with the use of such a light beam havebeen conventionally seen as sufficient for the requirements for formingswitching circuits and the like for pixel electrodes, for which suchhigh speed operation is not required.

However, in the case of integrally forming the elements requiring highspeed operation such as CPUs as well as a gate driving circuit and asource driving circuit on a single substrate, the polycrystalline thinfilms of the quality realized by the prior art technique areunsatisfactory. For example, the pixel region of LCDs generally requiresa mobility of about 0.5-10 cm²/Vs, whereas the peripheral drivingcircuits for controlling the pixels, such as gate circuits and sourcecircuits, require a mobility as high as about 100-300 cm²/Vs. In spiteof such requirements, the prior art technique utilizing the light beamhaving the above-described characteristic cannot achieve high mobilitiesconstantly. In other words, generally in polycrystalline silicon thinfilms, higher transistor characteristics are attained as the crystalgrain size is increased, but the above-described polycrystallizationtreatment cannot attain sufficient transistor characteristics.

The reason is that when the light beam having the above-describedcharacteristic is employed, uneveness in crystal grains andcrystallinity is increased, and in addition, when the irradiationintensity and the number of times of the irradiation are increased toimprove the crystallization, the crystal grain sizes become more uneven,further varying the crystallinity. The cause of this problem will now bediscussed in detail below.

FIG. 5 schematically shows the distribution of crystallinity in the casewhere the above-described rectangular light beam is applied to anamorphous silicon thin film formed on a substrate. In FIG. 5, thenumeral 1701 refers to the boundary of the irradiation beam, thenumerals 1702 and 1704 refer to the portions where the crystallinity islow, and the shaded portion 1703 indicates the portion where thecrystallinity is high. As shown in FIG. 5, by the prior art methodutilizing an excimer laser having a uniform energy intensity, thecrystallinity shows such a distribution pattern that only the shadedportion 1703 slightly inside the boundary of the irradiation light showshigh crystallinity, whereas the crystallinity is low in the otherportions (the adjacent portion 1702 to the boundary, and the centralportion 1704) show low crystallinity. This has been confirmed bymicro-Raman spectroscopy.

FIG. 6 shows the measurement result of Raman intensity on the portionalong the line A—A in FIG. 5. The abrupt peaks in FIG. 6 existingslightly inside the boundary indicate that the crystallinities in theseportions are high. In addition, the absence of peaks in the centralportion shows that the crystallinity in the central portion is low.

Referring now to FIG. 7, the mechanism of such uneveness incrystallinity will be discussed below. An amorphous silicon thin film isirradiated with a light beam to heat the thin film so that thetemperature of the thin film is increased above the melting pointtemperature of silicon (about 1400° C. or higher), and thereafter thelight irradiation is stopped. Thereby, the temperature of the thin filmdecreases by heat dissipation, and in this process, the fused siliconcrystallizes. Here, when the light beam has a uniform distribution ofthe light intensity as in FIG. 7(a), the surface of the irradiated thinfilm shows a temperature distribution pattern as in FIG. 7(b). That is,a flat temperature region in which no temperature gradient is present isformed in the central portion, and an abrupt temperature gradient isformed in the peripheral regions since the heat escapes outside. In thiscase, when the temperature of the central portion is higher than themelting point of silicon, the temperature in the portions adjacent tothe intersection points of the temperature distribution curve 1901 andthe crystallization temperature line 1902 (adjacent portions to theboundary) reaches the crystallization temperature first after theirradiation is stopped. Therefore, crystal nuclei 1903 are formed in thevicinity of these portions (see FIG. 7(c)). In other words, thetemperature of the amorphous silicon film is increased at a temperaturehigher than the melting point, and when the amorphous silicon thin filmis fused and then solidified in the regions heated above the meltingpoint, crystallization occurs, resulting in polycrystatlization.Following this, as the temperature further decreases (FIG. 7(d)), thecrystallization further proceeds from the crystal nuclei 1903 as thestarting points towards the central portion, where the temperature hasnot yet reach the crystallization temperature (FIG. 7(e)). In the caseof using the light beam having a uniform energy intensity, thetemperature decreases in such a manner that no temperature gradient inthe surface direction in the central portion is present, as shown inFIGS. 7(b), 7(d), and 7(f). Accordingly, at a certain point during thetemperature decrease, a relatively wide region reaches thecrystallization temperature at one time (FIG. 7(f)), and the crystalnuclei can be formed at any point of the wide region 1904 with an equalprobability. Therefore, as shown in FIG. 7(g), micro crystal nuclei aresimultaneously formed on the entire surface of the region 1904, and as aresult, a poly-Si thin film made of a multiplicity of microcrystalgrains is formed. Such a poly-Si thin film inevitably has a largedensity of grain boundaries. Therefore, the degree of carriers beingcaught in the grain boundaries increases, reducing the field effectmobility. It is noted that in FIG. 7(c), the numeral 1900 represents across section of the thin film.

The above-described mechanism for the uneveness of crystallinity beingcaused also applies to the case of employing a line-like laser beam forthe irradiation, as shown in FIG. 8. FIG. 8(a) illustrates thedistribution of energy densities in the directions x and y of an excimerlaser to be used. FIG. 8(b) shows the distribution of the increasedtemperatures of the amorphous silicon thin film irradiated with theexcimer laser having such energy densities. FIG. 8(c) is a perspectiveview of a polycrystalline silicon thin film transistor irradiated withthe laser as shown in FIGS. 8(a) and 8(b). As seen from these figures,since the laser having such an energy distribution as shown in FIG. 8(a)is employed, the region to be irradiated shows a temperaturedistribution almost uniform along the y direction, but shows such atemperature distribution in the x direction that the central portion ishigh and both side portions are low, as shown in FIG. 8(b). Due to sucha temperature distribution, the crystallization proceeds from theperipheral regions towards the central region along the x direction, andthe crystallization growth fronts of the numerous formed crystal nucleifrom both peripheral regions meet at the central region. Therefore, asshown in FIG. 8(c) which schematically illustrates the state of thecrystallization in the polycrystalline silicon thin film, although thecrystal grain size is large in the regions where the line beam energydensity of the laser beam is low, the grain size becomes small in theregion where the energy density is high (the central region). Forreference, in FIG. 8 (c), the numeral 131 designates the transparentinsulating substrate, the numeral 134 the polycrystal silicone thinfilm, and the numeral 141 the crystal grains. The numeral 139 refers toan insulating film which generally composed of a silicon dioxide (SiO₂)film, and the numeral 140 represents the amorphous silicon thin film.

Although, for the sake of brevity, the above-described example describesa case where the energy beam is applied only one time, the sameexplanation applies to such cases that the energy beam is applied aplurality of times.

In addition to the difficulty in improving field effect mobilities,prior art laser annealing methods have other drawbacks of the difficultyin improving the uniformity in quality of semiconductor films and of thedifficulty in meeting both these requirements.

Referring now to FIG. 9, a prior art leaser annealing apparatus isdescribed below. In FIG. 9, the numeral 151 designates a laseroscillator, the numeral 152 a reflector, the numeral 153 a homogenizer,the numeral 154 a window, the numeral 155 a substrate having anamorphous silicon layer formed thereon, the numeral 156 a stage, and thenumeral 157 a control unit. The laser annealing apparatus is soconstructed that a laser light emitted from the laser oscillator 151 isguided to the homogenizer 153 by the reflector 152, and the laser beamshaped by the homogenizer 153 in a predetermined shape with a uniformenergy is applied through the window 154 onto the substrate 155 fixed onthe stage 156 in the treatment chamber.

When performing an annealing treatment with the use of theabove-described annealing apparatus, because it is difficult toirradiate the entire substrate surface with a laser beam at one time,the regions to be irradiated is in turn staggered so that the region tobe irradiated overlaps with the already irradiated region, in order toirradiate the entire substrate surface under the same condition. See forexample, I. Asai, N. Kato, M. Fuse, and T. Hamano, Jpn. J. Apl. Phys. 32(1993) p.474. However, in such a laser annealing method, in which thelaser beam is applied in such a manner that the regions to be irradiatedare in turn staggered so that the region to be irradiated overlaps withthe already irradiated region, increasing the laser energy density canlead to an increase in the mobility, which is one of the evaluationstandards for semiconductor film characteristics, thereby increasing thefilm quality as a whole. However, the increased laser energy densityalso increases non-uniformity of the film quality at the overlappedregions, thus degrading the uniformity of the semiconductor film as awhole. On the other hand, when a relatively low energy density isemployed in the laser irradiation, improving the uniformity of the filmquality becomes easier, but because of the low energy density,increasing the field effect mobility becomes difficult.

Accordingly, in the case of employing a substrate having TFTs formedthereon for a liquid crystal display as schematically shown in FIG. 10,it has been difficult to form a semiconductor film which satisfies bothuniformity of the film quality, required for an image display region 158having a relatively large area, and field effect mobilities, requiredfor a peripheral circuit (driver circuit) region 159. It is to be notedhere that as a solution to this problem, U.S. Pat. No. 5,756,364suggests that the image display region 158 and the peripheral circuitregion 159 be irradiated with laser beams with different intensities.However, by merely varying the laser beam intensities, it is stilldifficult to obtain a field effect mobility sufficient for theperipheral circuit region 159.

Hence, as has been described above, prior art laser annealing methodshave the following drawbacks. Controlling crystal grain sizes andcrystal orientations is difficult and thereby forming semiconductor thinfilms having high crystal quality, i.e., large and uniform crystal grainsizes and few crystal defects. In addition, the reduction inmanufacturing cost by increasing throughputs is difficult. Furthermore,prior art methods cannot achieve the improvement in film characteristicsof semiconductor thin films (field effect mobilities etc.) and theuniformity of the film quality at the same time.

In view of the foregoing and other problems of prior art, it is anobject of the present invention to provide a method of producing asemiconductor thin film that can achieve a semiconductor thin filmhaving a high crystal quality without sacrificing throughputs, and canalso achieve both an improvement in film characteristics and auniformity in film quality in the semiconductor thin film at the sametime.

It is another object of the present invention to provide an apparatusfor producing such a semiconductor thin film.

It is further another object of the present invention to provide a thinfilm transistor having excellent TFT characteristics such as fieldeffect mobilities and the like, by utilizing the semiconductor thinfilm.

It is yet another object of the present invention to provide a methodfor producing such a thin film transistor.

It is to be noted here that the term “crystallization” as used hereinmeans both single crystallization and polycrystallization, and that amethod for producing a crystalline semiconductor thin film according tothe present invention is particularly useful in producing poly-Si thinfilms.

DISCLOSURE OF THE INVENTION

In view of the foregoing and other problems of prior art, it is anobject of the present invention to provide a method of producing asemiconductor thin film that can achieve a semiconductor thin filmhaving a high crystal quality without causing the reduction inthroughputs, and can also achieve both an improvement in filmcharacteristics and a uniformity in film quality in the semiconductorthin film at the same time.

It is another object of the present invention to provide an apparatusfor producing such a semiconductor thin film.

It is further another object of the present invention to provide a thinfilm transistor having excellent TFT characteristics such as fieldeffect mobilities and the like, by utilizing the semiconductor thinfilm.

It is yet another object of the present invention to provide a methodfor producing such a thin film transistor.

In order to provide a solution to the foregoing problems, the presentinventors have, as a result of intensive studies, found a method offorming at least a region in which a transistor is formed into apolycrystalline silicon thin film having a large grain size, based onthe consideration that a cause of crystal grains in a polycrystallinesilicon thin film being undesirably small is a temperature variation inthe silicon thin film heated by irradiating with an excimer laser.

More specifically, the present inventors have reached the followingidea; when performing a polycrystallization treatment by laser, byproviding a region having a high thermal conductivity on both sides ofthe region in which a transistor is formed so as to sandwich thetransistor-forming region, the temperature of the peripheral regionssandwiching the transistor-forming region is made higher than thetemperature of the transistor-forming region, which makes thetemperature of the transistor forming region lower relative to theperipheral regions, and thereby the silicon film in thetransistor-forming region is initially crystallized to increase crystalgrain sizes.

Accordingly, in accordance with a first aspect of the invention, thereis provided a method of producing a semiconductor thin film comprisingthe steps of stacking on a substrate a first insulating film having afirst thermal to conductivity and a second insulating film having asecond thermal conductivity being different from the first thermalconductivity, the second insulating film selectively formed in a partialregion on the substrate, stacking a non-single crystal semiconductorthin film over the first insulating film and the second insulating film,and irradiating the non-single crystal semiconductor thin film with anenergy beam to effect a crystal growth.

Specifically, for example, in an insulating film under an amorphoussilicon thin film, a thermal conductivity of a region in which atransistor is formed is made different from a thermal conductivity ofother regions, and thereby a thermal conductive performance of theamorphous silicon thin film in the region in which a transistor isformed is made higher than that of the amorphous silicon thin film inthe other regions.

By employing this method, in the polycrystallization, the temperature ofthe silicon thin film in the region where a transistor is formed islower than that of the other regions, and as a result, crystallizationstarts from the region in which a transistor is formed. Consequently, agrain size of the polycrystalline silicon in the region in which atransistor is formed can be made large.

In accordance with another aspect of the invention, in at least a partof a peripheral edge of the semiconductor thin film, at least oneprotruding part extending towards a horizontal direction with respect tothe semiconductor film may be provided.

Now, for a better understanding of the present invention, an approachwhich the inventors have taken to reach the present invention isdescribed below. First, the present inventors made strenuous efforts inorder to discover the cause of the foregoing problems in prior art, andreached the following factors as the cause. Generally, the generation ofcrystal nuclei and the crystal growth are effected by heating asemiconductor film by an annealing treatment and thereafter cooling thesemiconductor film. In the prior art, the semiconductor film is almostuniformly cooled after the annealing treatment regardless of whether itis in the central region or in the peripheral region, and as a result,crystal nuclei start to develop at random positions almost at one time.It is considered that this makes it difficult to control crystal grainsizes and crystal orientations. Also for the same reason, crystal nucleistart to develop at relatively adjacent positions almost at one time andthereby the crystals tend to interfere with each other in the process ofcrystal growth. This makes it difficult to obtain a sufficient crystalgrain size.

Based on the above factors, the present inventors have made strenuousstudies and reached a technical idea of the present invention that “theformation of crystal nuclei in the peripheral region in thesemiconductor film is started earlier than the formation of crystalnuclei in the central region, and thereafter the crystal nuclei formedin the peripheral region are grown towards the central region beforecrystal nuclei start to form or grow, in order to control crystal grainsizes and crystal orientations and to obtain a sufficient crystal grainsize by preventing the crystals undergoing crystal growth frominterfering with each other.”

More specifically, in accordance with this aspect of the invention, inthe semiconductor film after the annealing treatment, the heataccumulated in the protruding part in the peripheral edge diffuses in aplurality of outward directions with regard to a horizontal plane (forexample, in three directions in the case of the protruding part having arectangular shape), whereas the heat accumulated in the central regioncan escape, with regard to a horizontal plane, only in the directionstowards the peripheral edge, which has not yet been cooled, andtherefore the peripheral edge region, including the protruding part, canbe cooled sufficiently earlier than the central region.

Accordingly, the crystal nuclei in the peripheral edge region start toform earlier than those in the central region, and the crystal nuclei inthe peripheral region grow towards the central region before crystalnuclei are formed or grown in the central region, which makes itpossible to control crystal grain sizes and crystal orientations.Thereby, it is prevented that the crystals in the process of crystalgrowth interfere with each other, and a sufficient crystal grain sizecan be obtained.

In another embodiment of this aspect of the invention, the protrudingpart may have a size such that one crystal nucleus is formed whencrystal growth takes place by irradiating with the energy beam.Accordingly, only one crystal nucleus is formed in the protruding part,and the crystal nucleus is grown to be a crystal. In still anotherembodiment of this aspect of the invention, a length of said protrudingpart in a direction of protruding is greater than a film thickness ofsaid semiconductor thin film, and equal to or less than 3 μm, or a widthof the protruding part in a width direction perpendicular to thedirection of protruding is greater than the film thickness of thesemiconductor thin film, and equal to or less than 3 μm. Accordingly,the crystal grain size is further finely adjusted, and it is ensuredthat one crystal nucleus is formed in each protruding part.

In another embodiment of this aspect of the invention, the semiconductorthin film may be formed in a shape having a pair of line segmentsopposed to each other; two or more of the protruding parts may be formedin each of the pair of line segments; and an interval of the protrudingparts next to each other in each of the pair of line segments may be setto be approximately equal to an interval of the opposing line segments.

Accordingly, the crystal nucleus formed and grown in the protruding partfurther grows towards the central region, and it is expected thatcrystal growth is effected such that both the crystals grown from theprotruding parts next to each other towards the central region and thecrystals grown from the protruding parts in the opposed line segmentstowards the central region are grown with a minimum interference witheach other.

In another embodiment of this aspect of the invention, there is provideda semiconductor device comprising a semiconductor thin film whereincrystals are grown by irradiating a non-single crystal semiconductorthin film with an energy beam, characterized in that a protruding partis formed in a peripheral edge region of the semiconductor thin film,the protruding part extending outwardly in the same plane as a plane ofthe semiconductor thin film.

In the above device, a thin film transistor having a source region, agate region, and a drain region each made of the semiconductor thin filmmay be formed, and the protruding part may be formed at least in aperipheral edge region of the gate region.

Accordingly, the protruding part is provided in the region correspondingto the gate electrode, and thereby a good electrical conductivity isobtained.

In another embodiment of the invention, there is provided a method ofproducing a semiconductor thin film, comprising the steps of forming anon-single crystal semiconductor thin film having a protruding partextending outwardly in the same plane as a plane of the non-singlecrystal semiconductor thin film, and growing crystals in the non-singlecrystal semiconductor thin film by irradiating with an energy beam.

In a semiconductor film produced according to the above-describedmethod, the same advantageous effects as described above are attained.

In another embodiment of the invention, crystal nuclei in a peripheralregion in the non-single crystal semiconductor thin film are formedearlier than crystal nuclei in a central region in the non-singlecrystal semiconductor thin film, and thereafter, the crystal nuclei inthe peripheral region are grown towards the central region before thecrystal nuclei in the central region start to be formed or grown.Thereby, it is made possible to control crystal grain sizes and crystalorientations.

This achieves a sufficient crystal grain size since it is prevented thatcrystals in the process of crystal growth interfere with each other.

In order to the foregoing and other problems in prior art, the presentinvention also provides a crystalline thin film transistor including acrystalline semiconductor layer formed on a substrate, the crystallinesemiconductor layer comprising a channel region, a source regiondisposed at both sides of the channel region, and a drain region,wherein the crystalline semiconductor layer is such that a non-singlecrystalline thin film is crystallized, and at least in the channelregion in the crystalline semiconductor layer, a gap for controlling anorientation of crystal growth is provided.

In the above-described configuration, a gap for controlling anorientation of crystal growth formed in the channel region controls anorientation of crystal growth in the channel region when the non-singlecrystalline thin film is crystallized. Accordingly, in the crystallinesemiconductor layer having such a gap for controlling an orientation ofcrystal growth, shapes and density of grain boundaries of the crystalsare preferably controlled, and hence the crystalline thin filmtransistor exhibits excellent TFT characteristics, such as field-effectmobility.

It is to be noted here that the gaps for controlling an orientation ofcrystal growth is a depressed part formed on the surface of crystallinesemiconductor layer (non-single crystalline thin film in the process offabrication), and the depressed part may extend to a lower layer underthe crystalline semiconductor layer (the surface of the substrate or theundercoat layer) or may not extend to the lower layer. The sizes andshapes thereof are not particularly restricted, and therefore may besuitably adjusted depending on the surface area and thickness of thecrystalline semiconductor layer, a desired field effect mobility, and soforth. Examples of the surface shapes thereof include a circular shape,a square-shaped hole, a long and narrow groove, and so forth, andexamples of the cross-sectional shape thereof include a C-like shape, aV-like shape, and an angular C-like shape. The detail of the gap forcontrolling an orientation of crystal growth will be described later.

In another embodiment of the invention, there is provided a crystallinethin film transistor including a crystalline semiconductor layer formedon a substrate, the crystalline semiconductor layer comprising a channelregion, a source region disposed at both sides of the channel region,and a drain region, wherein the crystalline semiconductor layer is suchthat a non-single crystalline thin film is crystallized, wherein the gapfor controlling an orientation of crystal growth is divided into two ormore arrays of groove-like gaps provided in a direction linking thesource region and the drain region.

In this configuration, two or more arrays of groove-like gaps functionsso that the orientation of crystal growth is guided in the directionlinking the source region and the drain region, and the resultingpoly-Si film becomes an aggregate of large crystal grains longitudinallyextending in the direction linking the source region and the drainregion. Such a poly-Si film has a small density of grain boundaries inthe direction linking the source region and the drain region andtherefore exhibits a high carrier mobility. In other words, thecrystalline thin film transistor having the above-describedconfiguration has excellent characteristics such as carrier mobilities.

Now, referring to FIGS. 11 and 20, there is detailed the reason why byproviding the gap for controlling an orientation of crystal growth,large crystal grains in which the orientation of crystal growth iscontrolled are obtained.

As shown in FIGS. 20(a) and 20(b), on a surface of non-singlecrystalline thin film, which is a precursor material of the crystallinesemiconductor layer, two or more arrays of groove-like gaps forcontrolling an orientation of crystal growth (denoted by the referencenumeral 411) are formed in the direction linking the source region andthe drain region, and thereafter an energy beam capable of beingabsorbed is applied to the thin film in a conventional manner. Theresulting temperature distribution on the surface of the thin film is asfollows; the gaps for controlling, the region adjacent thereto, and theperipheral edge region show a low temperature, and the main portion ofthe channel region (a region of the thin film where the gap forcontrolling an orientation of crystal growth is not formed) shows a hightemperature.

The reason is that, since the groove region (gaps for controlling anorientation of crystal growth) has a smaller film thickness than theother regions or has no thin film thereon, less energy beam is absorbedin the groove region, and as a result the temperature of the grooveregion becomes lower than the other regions. In addition, normally,because no thin film exists outside the semiconductor thin film, whichleads to less absorbed energy beam, and also because heat diffusesoutward in the peripheral edge region, the temperature of the peripheraledge region becomes lower than the central region of the thin film.

There is now described below a process of crystal growth in thenon-single crystalline thin film having a temperature distribution suchthat the gap for controlling an orientation of crystal growth and theperipheral edge region show a low temperature. It is noted that in priorart as well, the peripheral edge region of non-single crystalline thinfilm shows a low temperature, and therefore the explanation hereconcerns with the relationship between orientations of crystal growthand the gap for controlling an orientation of crystal growth, withreference to FIGS. 11(a) and 11(b).

FIGS. 11(a) and 11(b) schematically illustrate a state of the crystalgrowth. First, crystal nuclei are formed in a peripheral region of thegap for controlling an orientation of crystal growth, where thetemperature is lower than that of the main portion. Then, the crystalnuclei grow in a direction towards a region having a higher temperature,i.e., in a direction away from the groove-like gap for controlling anorientation of crystal growth (a perpendicular direction to the groove),as the temperature of the whole thin film falls. Note here that in theabove-described configuration, two or more array of the gaps forcontrolling an orientation of crystal growth are provided in thedirection linking the source region and the drain region, and therefore,the crystal nuclei formed in the regions adjacent to the two gaps anorientation of crystal growth opposed to each other grow towards thecenter of the main portion of the channel region from the oppositedirections. Therefore, both crystal grains collide with each other inthe vicinity of the center of the main portion of the channel region.However, the central region, being far from the gap for controlling anorientation of crystal growth, has still a higher temperature than theother regions and therefore is in a state where molecules therein canstill freely move. Accordingly, the orientation of crystal growth isguided to the direction in which the collision is avoided, i.e., thedirection linking the source region and the drain region (the directionparallel to the groove, see FIG. 11a). As a result, a large crystalgrain is formed so as to longitudinally extend in the direction linkingthe source region and the drain region (see FIG. 11b ). When the channelregion is composed of an aggregate of the crystal grains having such ashape, the density of grain boundaries in the direction linking thesource region and the drain region becomes small, and therefore acrystalline thin film transistor having excellent TFT characteristicssuch as field effect mobility can be formed.

In another embodiment of the invention, the gap for controlling anorientation of crystal growth may be divided into a plurality of gapsdiscontinuously provided in a direction linking the source region andthe drain region.

When a plurality of gaps for controlling an orientation of crystalgrowth are discontinuously arranged, crystal growth is more finelycontrolled, and in particular, when two or more arrays of gaps forcontrolling. an orientation of crystal growth are arranged, grain sizesand shapes of the crystals are further finely controlled. The reasonsfor this are as follows.

As described above, crystal nuclei are formed in the vicinity of the gapfor controlling an orientation of crystal growth, in which thetemperature decreases to a crystallization temperature earlier. Here, ifthe intervals between the crystal nuclei are narrow, the crystal growthis hindered since crystals collide with other crystals before theysufficiently grow, and this results in a polycrystal made of amultiplicity of micro-crystal grains and a distorted crystal structurein the vicinity of the boundaries where crystal grains collide with eachother. For this reason, desired TFT characteristics cannot be obtained.Accordingly, in order to improve TFT characteristics such as fieldeffect mobility, it is necessary that the density of crystal nuclei tobe formed should be appropriately controlled, in addition to the controlof the orientation of crystal growth.

Here, if the gaps are arranged in a discontinuous manner, althoughcrystal nuclei are formed in the vicinity of the gaps, they are noteasily formed in intermediate regions between a gap and the next gap.Thus, by adjusting the number of the gaps and/or the intervals betweenthe gaps, the density of crystal nuclei to be formed can be controlled.It is noted that the reason why crystal nuclei are not easily formed inthe intermediate region between a gap and the next gap is that theintermediate region (the region where the thin film material is present)is sufficiently heated by laser irradiation.

In accordance with another aspect of the invention, there is provided a,semiconductor device including a crystalline semiconductor layer formedon a substrate, the crystalline semiconductor layer comprising a channelregion, a source region disposed at both sides of the channel region,and a drain region, the semiconductor device wherein the crystallinesemiconductor layer is such that a non-single crystalline thin film iscrystallized and at least in the channel region, anearly-crystallization region in which a crystallization-startingtemperature is higher than that in a main portion of the channel regionis provided.

By employing the above-described configuration, theearly-crystallization region serves to control the crystal growth in themain portion of the channel region, and as a result, high qualitycrystalline semiconductor layer having a small density of grainboundaries can be formed. The reason is as follows.

Because the crystallization-starting temperature is high in theearly-crystallization region, crystal nuclei start to be formed earliestin the early-crystallization region. The crystal nuclei become thecenter of the crystal growth to take place thereafter. Accordingly, byproviding the early-crystallization region, the phenomenon that multiplecrystal nuclei are formed at once can be prevented, and as a result, apolycrystalline semiconductor layer in which large crystal grains areaggregated can be formed.

It is preferable that the early-crystallization region be arranged atleast one or more in the channel region, and it is also preferable thata plurality of early-crystallization regions be provided at positionswhere the transfer of carriers in the direction linking is not hindered.When a plurality of early-crystallization regions are provided on thesurface of the thin film at appropriate positions and intervals, thedensity of crystal nuclei to be formed can be appropriately controlled,resulting in further desirable results. It is to be noted that thephrase “a crystallization-starting temperature is high” mentioned abovemeans that crystallization starts to take place at a higher temperaturethan that in the main portion of the channel region.

In another embodiment of the invention, the early-crystallization regionhas a shape longitudinally extending in a direction linking the sourceregion and the drain region.

Since the early-crystallization region is not a region where carriertransfer takes place, the region is preferable to have a narrow width inthe direction linking the source region and the drain region. If theearly-crystallization region extends longitudinally in the directionlinking the source region and the drain region, theearly-crystallization region can become a factor that hinders carriermobilities.

In another embodiment of the invention, the early-crystallization regionis such that an impurity is contained in a component constituting themain portion of the channel region.

In a technique of raising the crystallization-starting temperature byadding an impurity to the semiconductor layer, the early-crystallizationregion can be formed relatively easily. Accordingly, the crystallinethin film transistor having the above-described configuration not onlyexhibits excellent TFT characteristics such as field effect mobility butalso achieves a reduced cost.

In another embodiment of the invention, the crystalline semiconductorlayer is substantially composed of silicon or a compound of silicon andgermanium.

Silicon and a compound of silicon and germanium are readily availableand easy to crystallize. Accordingly, the above-described configurationachieves a high quality crystalline thin film transistor at a low cost.

The methods of producing a semiconductor device which will be describedbelow relate to the crystalline thin film transistors which has beendescribed above, and the advantageous effects by the methods are almostthe same as those described above. For this reason, the detailedexplanation for the advantageous effects will not be repeated below.

In accordance with another aspect of the invention, there is provided amethod of producing a crystalline thin film transistor including acrystalline semiconductor layer, the crystalline semiconductor layercomprising a channel region, a source region disposed at both sides ofthe channel region, and a drain region, the method comprising at leastthe steps of depositing a non-single crystalline thin film on aninsulating substrate, forming a plurality of gaps for controlling anorientation of crystal growth in the non-single crystalline thin film,and irradiating the thin film in which the plurality of gaps are formedwith an energy beam to crystallize the thin film.

In the above-described method, the gaps for controlling an orientationof crystal growth may be formed in a direction linking the source regionand the drain region so as to have a groove-like shape, and in addition,the gap for controlling an orientation of crystal growth may be dividedinto a plurality of gaps discontinuously formed in a direction linkingthe source region and the drain region. By employing these methods, theforegoing crystalline thin film transistors can be produced.

In accordance with another aspect of the invention, there is provided amethod of producing a crystalline thin film transistor including acrystalline semiconductor layer, the crystalline semiconductor layercomprising a channel region, a source region disposed at both sides ofthe channel region, and a drain region, the method comprising at leastthe steps of depositing a non-single crystalline thin film on aninsulating substrate, forming an early-crystallization region byion-implanting an impurity in a partial region in the non-singlecrystalline semiconductor thin film, the impurity for raising acrystallization-starting temperature of the partial region, and afterthe step of forming an early-crystallization region, irradiating thethin film with an energy beam to crystallize the thin film.

In the above-described method, the early-crystallization region may havea belt-like shape longitudinally extending in a direction linking thesource region and the drain region, and in addition, theearly-crystallization region is divided into a plurality ofearly-crystallization regions discontinuously disposed in a directionlinking the source region and the drain region. By employing thesemethods, the foregoing crystalline thin film transistors can beproduced.

In addition, in each of the foregoing producing methods, the energy beammay be an excimer laser beam.

Excimer lasers have a large light energy, and is well absorbed bysilicon since they are UV lights. Therefore, by using an excimer laserbeam, crystallization of the non-single crystalline semiconductor layercan be efficiently performed. In particular, when the non-singlecrystalline semiconductor layer is composed of a material capable ofabsorbing ultraviolet rays such as silicon, it is possible toselectively heat and fuse only the semiconductor layer. Therefore, thecrystallization of the semiconductor layer can be effected withoutcausing adverse effects by heat on the regions not irradiated with thebeam, and moreover, it is made possible to employ glass substrates,which are low in cost. Furthermore, when an excimer laser and a thinfilm material capable of absorbing UV are used in combination, thetemperature difference between the gap for controlling an orientation ofcrystal growth and the main portion of the semiconductor layer becomeslarge, and thereby the function of the gap for controlling anorientation of crystal growth (function for controlling the orientationof crystal growth) can be fully exploited.

The present inventors have also carried out a study on methods forsufficiently growing crystals based on the foregoing considerationsregarding the mechanism of crystallization. As a consequence, theinventors have found that by intentionally making uneven a distributionof the light intensity within the light beam width, crystallization cansmoothly progress, and thereby a high quality crystalline thin film canbe obtained. Based on this view, the following aspects of the presentinvention have been accomplished.

In accordance with another aspect of the invention, there is provided amethod of producing a semiconductor device wherein a thin film of anon-single crystalline material formed on a substrate is irradiated witha light beam whereby the non-single crystalline material is crystallizedor recrystallized to form a crystalline semiconductor thin film, themethod characterized in that the light beam is such that a distributionpattern of a light energy intensity of the light beam is adjusted sothat a temperature gradient or an uneveness of temperature distributionis caused, and the light beam is applied in a stationary state.

In the above-described method, uneveness of the temperature gradient ortemperature distribution is caused on the surface of the non-singlecrystalline thin film irradiated with the light beam, and thereby it ismade possible to prevent the phenomenon that micro crystal nuclei aresimultaneously formed in a wide region, the phenomenon explainedpreviously referring to FIGS. 7(f) and 7(g). Therefore, relatively largecrystal grains are obtained, and evenness in the degree of crystallinityis increased. Consequently, the density of grain boundaries becomessmall, and field effect mobility is improved.

In another embodiment of this aspect of the invention, a distributionpattern of the light energy intensity may be such a distribution patternthat a light intensity within a beam width monotonously increases fromone side to the other, or monotonously decreases from one side to theother.

In this configuration, the temperature gradient on the surface of thenon-single crystalline thin film to be irradiated is formedcorrespondingly to the light energy intensity, and the crystallizationis guided in a direction from a region where the temperature is lowtowards a region where the temperature is high. Thus, disorderlyformation of crystal nuclei and disorderly crystal growth are prevented,and consequently it is ensured that the phenomenon explained with FIGS.7(f) and 7(g) is prevented.

In the case of employing the crystalline thin film for, for example, asemiconductor circuit comprising a source region—a channel region—and adrain region, it is preferable that the intensity gradient of the lightenergy be formed in the direction parallel to the source-draindirection. Thereby, the direction of crystal growth is restricted to thedirection parallel to the direction of the transfer of carriers, and thedensity of grain boundaries becomes small. Accordingly, by employingthis technique, a mobility of, for example, 300 cm²/Vs or higher can beachieved.

In another embodiment of this aspect of the invention, a distributionpattern of the light energy intensity may be such that, in a beam width,a part having a relatively stronger light intensity and a part having arelatively weaker light intensity are alternately arrayed in a plane.

When a light beam having a striped pattern made of a part having astrong light intensity and a part having a weak light intensity isapplied, a striped temperature distribution pattern made of a parthaving a high temperature and a part having a low temperature is formedon the irradiated surface. In such a striped temperature distributionpattern, crystal growth is guided in the direction from a region wherethe temperature is low (normally formed in a belt-like shape) towards aregion where the temperature is high. Then, crystal grains collide witheach other in the vicinity of the center of the region (belt) where thetemperature is high, forming a continuous line of grain boundaries (acontinuous line like a mountain range) there, and crystal grains areformed so as to longitudinally extending in the direction parallel tothe continuous line.

Hence, in this configuration as well, the phenomenon explained withFIGS. 7(f) and 7(g) is prevented, and moreover, the same advantageouseffects as described in the above-described configuration employing suchan intensity distribution monotonously increasing or decreasing are alsoobtained. Specifically, crystallization is effected while arranging aregion with a relatively strong light intensity and a region with arelatively weak light intensity parallel to the source-drain direction.Thereby, the collision line of crystal grains becomes parallel to thesource-drain direction, and it is prevented that carriers cross thecollision line of crystal grains (the line of grain boundary), whichcauses a considerable decrease in the mobility. Thus, a channel regionhaving a high mobility can be formed.

In the above-described method, a distribution pattern of the lightenergy intensity may be formed by causing a light interference bysimultaneously irradiating with at least two coherent lights.

In this method utilizing a light interference, it is possible to form afine light intensity distribution, and as a result to form a finestriped temperature distribution on the surface to be irradiated.Accordingly, this method achieves a smooth crystallization in arelatively wide region.

In another embodiment of the invention, a distribution pattern of thelight energy intensity of may be a wave-motion-like interference patternformed by simultaneously irradiating with at least two coherent lightsand by dynamically modulating a phase of at lease one of the twocoherent lights.

In this method utilizing a dynamic light interference pattern, theenergy intensity distribution of the light beam varies in awave-motion-like manner, and the temperature of the irradiated surfacecorrespondingly varies in a wave-motion-like manner so that thetemperature moves in one direction. Accordingly, by employing thismethod, impurities contained in the non-crystalline thin film can begradually expelled outside the effective area, and consequently acrystalline thin film having a high purity and a high mobility can beformed.

It is noted that in the methods of producing a crystalline thin filmaccording to this aspect of the invention, the light beam may be appliedas the beam is being moved relative to the non-single crystalline thinfilm on the substrate. In this method in which a light beam is appliedwhile being moved relative to the surface of the thin film, the lightbeam having a light energy intensity distribution pattern being adjustedso that a temperature gradient or an uneveness of the temperaturedistribution is caused on the surface to be irradiated (the surface ofthe non-single crystalline thin film), the orientation of crystal growthcan be finely guided. Therefore, a high quality crystalline thin filmhaving a high uniformity in the degree of crystallinity and a smalldensity of grain boundaries can be obtained.

In accordance with another aspect of the invention, there is provided amethod of producing a semiconductor thin film wherein a thin filmcomprising a non-single crystalline material formed on a substrate isirradiated with a light beam and thereafter cooled whereby thenon-single crystalline material is crystallized or recrystallized, themethod characterized in that a pressure of an atmosphere gas ismaintained at more than a predetermined value to cause an uneventemperature distribution on a surface of the thin film irradiated withthe light beam.

In this method, at a moment when the molecules of the gas constitutingthe atmosphere gas collide with the surface of the thin film and detachtherefrom, the molecules deprive the thin film of heat, forming a lowtemperature region in a limited area. Thus, crystal nuclei are formed inthe region, and the formed crystal nuclei facilitate the crystal growth,consequently preventing the phenomenon explained with FIGS. 7(f), and7(g).

In the above-described method, the pressure of the atmosphere gas to bemaintained at more than a predetermined value may be 10⁻⁵ torr or higherwhere the atmosphere gas is a hydrogen gas.

When a laser annealing treatment is performed under a hydrogen gaspressure of 10⁻⁵ torr or higher, the above-described advantageous effectis ensured by the movement of hydrogen molecules, which have a highspecific heat.

In order to provide a solution to the foregoing and other problems, theinvention also provides a method of producing a semiconductor filmcomprising the step of crystallizing a precursor semiconductor film, thestep wherein the precursor semiconductor film formed on a substrate isirradiated with a first energy beam supplying the precursorsemiconductor film with at least such an energy that the precursorsemiconductor film can be crystallized, and with a second energy beamsuch that an absorption index of said precursor semiconductor film issmaller than an absorption index by said first energy beam and an energysupplied by said second energy beam is smaller than an energy capable ofcrystallizing said precursor semiconductor film.

According to this method, the second energy beam can easily reach alower portion of the precursor semiconductor film and further thesubstrate. Thereby, the precursor semiconductor film is heated throughthe thickness direction and the substrate is also heated, reducing thetemperature difference between the point while the first energy beam isbeing applied and the point after the irradiation with the beam iscompleted. Thus, the precursor semiconductor film heated and fused bybeing irradiated with the first energy beam is crystallized after theirradiation is completed, while being annealed. Therefore, the crystalgrowth is facilitated, and it is made possible to form relatively largecrystal grains and reduce crystal defects, which improves electricalcharacteristics of the semiconductor film.

In the above-described method, the precursor semiconductor film may bean amorphous silicon thin film.

Thereby, a polycrystalline silicon thin film having good crystal qualityand good electrical characteristics such as field effect mobility can bereadily produced.

Further in the above-described method, the first energy beam may be suchthat an absorption coefficient of the precursor semiconductor film isapproximately equal to or greater than the reciprocal of a filmthickness of the precursor semiconductor film, and the second energybeam may be such that an absorption coefficient of the precursorsemiconductor film is approximately equal to or less than the reciprocalof a film thickness of the precursor semiconductor film.

By employing this method, much of the first energy beam is absorbed inthe vicinity of the surface of the precursor semiconductor film, whereasmuch of the second energy beam reaches the lower portion of theprecursor semiconductor film and the substrate, and thus the precursorsemiconductor film is efficiently heated as well as the substrate. Thus,after the irradiation with the first energy beam is completed, theprecursor semiconductor film is annealed and the crystal growth isfacilitated. Therefore, it is ensured that relatively large crystalgrains are formed, and a semiconductor film having good crystal qualityis formed.

Further in the above-described method, the first energy beam may be suchthat an absorption coefficient of the precursor semiconductor film isapproximately 10 times or greater than the reciprocal of a filmthickness of the precursor semiconductor film, and the second energybeam may be such that an absorption coefficient of the precursorsemiconductor film is approximately the reciprocal of a film thicknessof the precursor semiconductor film.

By employing this method, the precursor semiconductor film can be moreefficiently heated, and a semiconductor film having further higherquality can be formed.

In the above-described method, the first and second energy beams have adifferent wavelength from each other.

By employing this method, the difference of the absorption coefficientsas described above can be readily realized.

The foregoing energy beams having a different wavelength from each othermay be, for example, such that the first energy beam is an energy beamhaving a single wavelength, and the second energy beam includes at leasta wavelength component in a visible light range.

More specifically, the first energy beam and the second energy beam maybe, for example, a laser light and an infrared lamp, a laser light andan incandescent light, or a laser light and an excimer lamp light.

In addition, as the foregoing lights having a different wavelength fromeach other, for example, the second energy beam may contain at least awavelength component from a visible light range to an ultraviolet range,such as a xenon flash lamp light.

In addition, the first energy beam and the second energy beam may be alaser light.

When the laser beam is employed, the irradiation with the energy beamhaving a large energy density can be readily performed, and thereby itis made easy to efficiently heat the precursor semiconductor film andthe substrate.

More specifically, for example, in the case where the precursorsemiconductor film is an amorphous silicon thin film, the first energybeam may be one laser light selected from an argon fluoride excimerlaser, a krypton fluoride excimer laser, a xenon chloride excimer laser,and a xenon fluoride excimer laser, and the second energy beam may be alaser light of an argon laser.

In addition, for example, in the case where the substrate is a glasssubstrate and the precursor semiconductor film is an amorphous siliconthin film, the first energy beam is one laser light selected from anargon fluoride excimer laser, a krypton fluoride excimer laser, a xenonchloride excimer laser, and a xenon fluoride excimer laser, and thesecond energy beam is a laser light of a carbon dioxide gas laser.

Each of the excimer lasers mentioned above is easy to obtain a largepower and is easily absorbed in the vicinity of the surface of theamorphous silicon thin film. The laser light of an argon laser transmitsthrough the amorphous silicon film to a certain degree and is easilyabsorbed throughout the thickness direction of the amorphous siliconthin film. The carbon dioxide gas laser transmits through the amorphoussilicon thin film relatively well and is easily absorbed by the glasssubstrate. Hence, the amorphous silicon thin film can be efficientlyheated, a polysilicon thin film having good crystal quality can bereadily formed, and the productivity can be readily improved.

In the method according to this aspect of the invention, the firstenergy beam and the second energy beam may be applied to a belt-likeshaped region in the precursor semiconductor film.

By applying the beams to the belt-like shaped region, heating can beperformed with a uniform temperature distribution, and thereby it ismade possible to easily form a semiconductor film having a uniformcrystal quality and to reduce the time required for the process ofcrystallization.

In the method according to this aspect of the invention, a region in theprecursor semiconductor film to be irradiated with the second energybeam may be larger than a region in the precursor semiconductor film tobe irradiated with the first energy beam, and may include the region tobe irradiated with the first energy beam.

In this method as well, heating can be performed with a uniformtemperature distribution, and thereby it is made possible to easily forma semiconductor film having a uniform crystal quality.

In the method according to this aspect of the invention, the firstenergy beam and the second energy beam are incident approximatelyperpendicular to the precursor semiconductor film.

When each of the energy beams are incident approximately perpendicularto the precursor semiconductor film as described above, a variation inthe irradiation with each energy beam is reduced, and thereby it is madepossible to easily form a semiconductor film having a uniform crystalquality.

In the method according to this aspect of the invention, the secondenergy beam is applied at least prior to applying the first energy beam.Such applying of the second energy beam prior to applying the firstenergy beam may be performed by controlling the timings of applying ofthe energy beams, and further, may be performed in such a manner thatthe substrate on which the precursor semiconductor film is formed ismoved, and the second energy beam is applied to a more forward positionin the precursor semiconductor film with respect to a direction ofmoving of the substrate than a position where the first energy beam isapplied.

By performing such applying of the energy beams, crystallization iseffected by the first energy beam in a state where the semiconductorfilm and the substrate are sufficiently heated by the second energybeam, resulting in an efficient crystallization process.

In the method according to this aspect of the invention, the firstenergy beam may be intermittently applied, and the second energy beammay be continuously applied.

Specifically, the first energy beam may be a pulsed laser light, and thesecond energy beam may be a continuous-wave laser light or a lamp light.

By continuously applying the second energy beam as described above, itis made easy to heat the substrate and the precursor semiconductor filmat a predetermined stable temperature, and in addition, byintermittently applying the first energy beam, heat conduction to thesubstrate is suppressed to prevent the fusion or distortion of thesubstrate caused by overheating the substrate. Thereby it is ensuredthat the crystallization of the precursor semiconductor film can bereadily attained.

In the method according to this aspect of the invention, the firstenergy beam and the second energy beam may be synchronized with eachother and intermittently applied. Specifically, as the timings of theirradiation, it is preferable that a time of irradiating with the firstenergy beam should be within a time of irradiating with the secondenergy beam, and should be two-thirds or shorter of an irradiation cycleof the second energy beam. For the energy beams, specifically, the firstenergy beam may be a pulsed laser light, and the second energy beam maybe a pulsed laser light or an intermittently-operated lamp light.

By intermittently applying the first energy beam and the second energybeam as described above, it is easily made possible to irradiate a unitarea with a large light energy, and therefore heating with a largeenergy can be performed while preventing the fusion and distortion ofthe substrate caused by overheating the substrate, which easily ensuresthe crystallization of the precursor semiconductor film. In particular,the pulsed laser can easily attain a large power and thereby readilyheat a large area at a high temperature. Therefore, the time requiredfor the step of crystallization can be easily reduced to improveproductivity.

In the method according to this aspect of the invention, the firstenergy beam and the second energy beam may be applied so that theprecursor semiconductor film is heated at a temperature of 300° C. to1200° C., or more preferably at a temperature of 600° C. to 1100° C.

By heating the precursor semiconductor substrate at the temperature inthe above-described range, the temperature variation in crystallizationis made gentle and the crystal growth is facilitated while preventingcrystal defects and uneven crystallization caused by the formation ofmicro-crystals in a partial region, and the formation of large crystalgrains are readily made possible.

In addition, the method according to this aspect of the invention mayfurther comprise a step of heating the substrate on which the precursorsemiconductor film is formed with a heater. More specifically, forexample, it is preferable that the substrate on which the precursorsemiconductor film is formed should be heated at a temperature of 300°C. to 600° C.

By heating the substrate with the heater in addition to the secondenergy beam, the precursor semiconductor substrate, is more efficientlyheated, and in addition, the crystal growth is readily facilitated byannealing. Moreover, in comparison with the conventional case of heatingthe substrate with the heater only, a predetermined heating temperaturecan be obtained within a shorter time, which easily achieves animprovement in productivity.

In the method according to this aspect of the invention, the firstenergy beam may be applied to a plurality of regions in the precursorsemiconductor film, and the second energy beam may be applied to only apart of the plurality of regions.

By applying the second energy beam to only a partial region,crystallinity can be improved in a limited region, for example, whichrequires particularly high electrical characteristics, and therefore anecessary and sufficient crystallization can be effected by acrystallization process with a short time, which easily achieves animprovement in productivity.

In the method according to this aspect of the invention, the secondenergy beam may be such that an absorption index of the substrate islarger than an absorption index of the precursor semiconductor film. Inaddition, it is preferable that the first energy beam be such that anabsorption coefficient of the precursor semiconductor film isapproximately 10 times or greater than the reciprocal of a filmthickness of the precursor semiconductor film.

More specifically, in the case where the substrate is a glass substrate,the precursor semiconductor film is an amorphous silicon thin film, thefirst energy beam may be one laser light selected from an argon fluorideexcimer laser, a krypton fluoride excimer laser, a xenon chlorideexcimer laser, and a xenon fluoride excimer laser, and the second energybeam may be a laser light of a carbon dioxide gas laser.

By employing this method, much of the first energy beam is absorbed inthe vicinity of the surface of the precursor semiconductor film, whereasmuch of the second energy beam is absorbed by the substrate, and thusthe precursor semiconductor film is efficiently heated as well as thesubstrate. Thus, after the irradiation with the first energy beam iscompleted, the precursor semiconductor film is annealed and the crystalgrowth is facilitated. Therefore, it is ensured that relatively largecrystal grains are formed, and a semiconductor film having good crystalquality is formed.

In accordance with another aspect of the invention, there is provided anapparatus for producing a semiconductor film by crystallizing aprecursor semiconductor film formed on a substrate, comprising a firstirradiating means emitting a first energy beam, and a second irradiatingmeans emitting a second energy beam resulting in a smaller absorptionindex of said precursor semiconductor film than said first energy beam.

By employing the above-described apparatus, the second energy beam caneasily reach a lower portion of the precursor semiconductor film andfurther the substrate. Thereby, the precursor semiconductor film isheated through the thickness direction and the substrate is also heated,reducing the temperature difference between the point while the firstenergy beam is being applied and the point after the irradiation withthe beam is completed. Thus, the precursor semiconductor film heated andfused by being irradiated with the first energy beam is crystallizedafter the irradiation is completed, while being annealed. Therefore, thecrystal growth is facilitated, and it is made possible to formrelatively large crystal grains and reduce crystal defects, whichimproves electrical characteristics of the semiconductor film.

In the above-described apparatus according to this aspect of theinvention, the second irradiating means is a lamp radially emitting thesecond energy beam, and the apparatus further comprises a concavereflector collecting the second energy beam.

By employing the above-described apparatus, efficient heating of thesubstrate and so forth is easily performed with a uniform temperaturedistribution, and thereby it is made possible to easily form asemiconductor film having a uniform crystal quality.

The above-described apparatus for producing a semiconductor film mayfurther comprise a reflective plate in which one of the first energybeam and the second energy beam is reflected and the other one of thefirst energy beam and the second energy beam is allowed to transmit, theapparatus wherein the first energy beam and the second energy beam areincident perpendicular to the precursor semiconductor film.

When each of the energy beams are incident approximately perpendicularto the precursor semiconductor film as described above, a variation inthe irradiation with each energy beam is reduced, and thereby it is madepossible to easily form a semiconductor film having a uniform crystalquality.

In the above-described apparatus, specifically, in the case where theprecursor semiconductor film is an amorphous silicon thin film, thefirst irradiating means may be one of an argon fluoride excimer laser, akrypton fluoride excimer laser, a xenon chloride excimer laser, and axenon fluoride excimer laser, and the second irradiating means may be anargon laser.

In addition, in the case where the substrate is a glass substrate andthe precursor semiconductor film is an amorphous silicon thin film, thefirst energy beam may be one laser light selected from an argon fluorideexcimer laser, a krypton fluoride excimer laser, a xenon chlorideexcimer laser, and a xenon fluoride excimer laser, and the second energybeam may be a laser light of a carbon dioxide gas laser.

In order to provide a solution to the foregoing and other problems ofprior art, the present invention also provides, in another aspect of theinvention, a method of producing a semiconductor thin film comprising astep of irradiating a non-single crystal semiconductor thin film with anenergy beam, said non-single crystal semiconductor thin film formed on asubstrate having an image display region and a driving circuit region,said method characterized in that a first irradiation of said imagedisplay region is performed by using an energy beam having a line-likecross-sectional shape, and a second irradiation of said driving circuitregion is performed at a higher energy density than said firstirradiation by using an energy beam having a square-like cross-sectionalshape.

The present invention also provides a method of producing asemiconductor thin film comprising a step of irradiating a non-singlecrystal semiconductor thin film with an energy beam, said non-singlecrystal semiconductor thin film formed on a substrate having an imagedisplay region and a driving circuit region, said method characterizedin that a first irradiation of said image display region is a scanningirradiation such that said substrate is scanned by said energy beam in arelative manner and a region to be irradiated with said energy beam isshifted with a predetermined overlap, and a second irradiation of saiddriving circuit region is a stationary irradiation with a higher energydensity than said first irradiation such that said energy beam is fixedwith respect to said substrate in a relative manner.

Specifically, for example, in the thin film transistors constituting aliquid crystal display device, different laser irradiation methods areemployed for a pixel region which requires a uniformity of semiconductorfilm characteristics and for a driving circuit region which requirescharacteristics (particularly high mobility). Specifically, whenperforming a laser annealing in which amorphous silicon is irradiatedwith a laser light so as to fuse and crystallize the amorphous siliconto form polycrystalline silicon, the energy density of the laser lightapplied to the driving circuit region in the substrate plane is madehigher than the energy density of the laser light applied to the pixelregion, so as to form polycrystalline silicons in the driving circuitregion and in the pixel region each having different characteristicsfrom each other. More specifically, for example, a first laser lightirradiation is performed for the pixel region alone or for the entiresurface of the substrate, and thereafter a second laser lightirradiation is performed for the driving circuit region using a laserlight having a higher energy density than the laser light used in thefirst laser light irradiation.

According to this method, the mobility of the polycrystalline silicon inthe driving circuit region becomes higher than the mobility of thepolycrystalline silicon in the pixel region, and the characteristics ofthe polycrystalline silicon in the pixel region can be made uniform inthe plane.

In the above-described method, the first irradiation may be performed byusing an energy beam having a line-like cross-sectional shape, and thesecond irradiation may be performed by using an energy beam having asquare-like cross-sectional shape. Thereby, the laser annealing can beperformed without rotating 90 degrees the stage for fixing thesubstrate.

Additionally, in the above-described method, the first irradiation maybe a scanning irradiation such that a laser beam is applied a pluralityof times while a position to be irradiated with the laser beam is beingshifted, and the second irradiation may be a stationary irradiation suchthat the position to be irradiated is fixed. Thereby, the mobility ofthe polycrystalline silicon can be increased, and in addition, theuniformity is attained.

Further, the laser annealing may be performed in such a manner that aplurality of regions in the driving circuit region are irradiated withlaser lights having different energy densities from each other, therebyforming polycrystalline silicons having different characteristics fromeach other. In this case, it is preferable that the laser annealing beperformed so that a region in which a transfer gate in the latch orshift resistor and the other regions are respectively irradiated withlaser lights having different energy densities.

Further, in the above-described laser annealing methods, it ispreferable that the edge of the laser beam not fall on the TFT pattern.

In accordance with another aspect of the invention, there is provided anapparatus for producing a semiconductor thin film comprising an energybeam generating means, and means for homogenizing an energy beam emittedfrom the energy beam generating means by shaping the energy beam so asto have a predetermined cross-sectional beam shape and a homogeneousenergy, the apparatus wherein a non-single crystal semiconductor thinfilm formed on a substrate is irradiated with the energy beam to effectcrystal growth the apparatus characterized in that the apparatus furthercomprises a filter having a plurality of transmissivities different fromeach other, and the energy beam is applied through the filter to aplurality of regions in the non-single crystal semiconductor thin filmin such a manner that each of the plurality of regions receives adifferent energy density from each other.

By employing this method, it is made possible to form a plurality ofpolycrystalline semiconductor film having different characteristics on asingle substrate plane.

In the above-described method, the apparatus m ay be a laser annealingapparatus in which transmissivities of the mask are varied by an opticalthin film so as to have the plurality of transmissivities different fromeach other, and this enables the distribution of transmissivities toaccurately formed. In addition, the apparatus may be a laser annealingapparatus in which the mask and the window for applying the laser lightto the substrate in the treatment chamber are integrated, and thisenables the apparatus to have a simple construction and to reduceattenuation of the light energy.

In another embodiment of the invention, there is provided an apparatusfor producing a semiconductor thin film comprising an energy beamgenerating means, and means for homogenizing an energy beam emitted fromthe energy beam generating means by shaping the energy beam so as tohave a predetermined cross-sectional beam shape and a homogeneousenergy, the apparatus wherein a non-single crystal semiconductor thinfilm formed on a substrate is irradiated with the energy beam to effectcrystal growth, the apparatus characterized in that, the means forhomogenizing is capable of selectively shaping the energy beam into aplurality of cross-sectional beam shapes.

By employing this method, a laser light with the most appropriate shapeis applied to each position on the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a transmissivity characteristic of anamorphous silicon thin film.

FIG. 2(a) is a schematic plan view showing a prior art thin filmtransistor (TFT), and FIG. 2(b) is a schematic cross sectional viewthereof.

FIG. 3 illustrates a prior art method of producing a polysilicon thinfilm.

FIG. 4 illustrates a pattern of an intensity of a light beam having aflat light intensity distribution according to a prior art technique.

FIG. 5 is a schematic view illustrating non-uniformity of a degree ofcrystallinity in a crystallized region in a prior art technique.

FIG. 6 is a graph of a Raman intensity curve in the line A—A in FIG. 5.

FIGS. 7(a) to 7(g) illustrate progress of crystallization in the case ofemploying a light beam having a flat light intensity distribution.

FIGS. 8(a) to 8( c) illustrate the principle of polycrystailization bylaser light irradiation in accordance with a prior art method.

FIG. 9 is a schematic view showing a prior art laser anneal apparatus.

FIG. 10 illustrates a laser-annealed region in a liquid crystal display.

FIGS. 11(a) and (b) illustrate an orientation of crystal growth in ana-Si film provided with gaps for controlling an orientation of crystalgrowth.

FIGS. 12(a) to 12( c) illustrate a principle of polycrystallization inExample 1-1.

FIG. 13 is a graph showing a degree of crystallinity in apolycrystalline silicon thin film of Example 1-1.

FIG. 14(a) is a plan view of a TFT of Example 1-2, and FIG. 14(b) is across sectional view thereof.

FIG. 15 is a plan view of a TFT of Example 1-3.

FIG. 16(a) is a plan view of a TFT of Example 2-1, and FIG. 16(b) is across sectional view thereof.

FIG. 17(a) is a plan view of a TFT of Example 2-2, and FIG. 17(b) is across sectional view thereof.

FIGS. 18(a) to 18( e) illustrate production steps for the TFT of Example2-1.

FIGS. 19 (a) to 19( e) illustrate production steps for the TFT ofExample 2-2.

FIGS. 20(a) and 20(b) are a plan view and a cross sectional view showinga construction of a TFT of Example 3-1.

(FIG. 20(a) is the plan view, and FIG. 20(b) is a cross sectional viewtaking along the line A-A′ in FIG. 20(a).)

FIG. 21 is a cross sectional view taken along the line B-B′ in FIG.20(a) of Example 3-1.

FIGS. 22(a) to 22( e) illustrate production steps for the TFT of Example3-1.

FIGS. 23(a) and 23(b) are a plan view and a cross sectional view showinga construction of a TFT of Example 3-2.

(FIG. 23(a) is the plan view, and FIG. 20(b) is a cross sectional viewtaking along the line C-C′ in FIG. 23(a).)

FIG. 24 is a cross sectional view taken along the line D-D′ in FIG.23(a).

FIGS. 25(a) to 25(e) illustrate production steps for the TFT of Example3-2.

FIG. 26 is a plan view showing a construction of a TFT of Example 3-3.

FIG. 27 is a plan view showing a construction of a TFT of a variation inaccordance with Examples 3-1 to 3-3.

FIG. 28 is a cross sectional view showing a construction of a TFT ofanother variation in accordance with Examples 3-1 to 3-3.

FIGS. 29(a) to 29( g) illustrate progress of crystallization in the caseof employing a light beam having a light intensity gradient.

FIG. 30 is a schematic view illustrating a state of moving and applyinga light beam having a light intensity gradient.

FIG. 31 illustrates a light transmission characteristic of a filter forproducing a light beam having a light intensity gradient.

FIGS. 32(a) to 32( g) illustrate progress of crystallization in the caseof employing a light beam in which a region having a relatively stronglight intensity and a region having a relatively weak light intensityare alternatively arrayed with respect to a plane.

FIG. 33 is a schematic view illustrating a state of moving and applyinga light beam having a distribution pattern shown in FIG. 32(a).

FIG. 34 shows a light transmission characteristic of a filter forproducing the light beam as shown in FIGS. 32(a) to 32(g).

FIG. 35 shows a distribution pattern of light intensity of anotherexample of the light beam in which a region having a relatively stronglight intensity and a region having a relatively weak light intensityare alternatively arrayed with respect to a plane.

FIG. 36 is a schematic view illustrating the principle of producing thedistribution pattern of light intensity shown in FIG. 35 by lightinterference.

FIGS. 37(a) to 37( g) illustrate progress of crystallization in the caseof employing the light beam of FIG. 35.

FIG. 38 is a schematic view illustrating a method of producing a lightbeam by a dynamic interference pattern in which bright-line parts anddark-line parts propagate.

FIG. 39 is a schematic view showing a state of a light interferencepattern formed in a thickness direction of the thin film.

FIG. 40 is a schematic view showing a state of heat flowing outwardlyfrom the thin film heated by light irradiation.

FIG. 41 illustrates relationships between ambient pressures in lightirradiation and numbers of irradiation and degrees of crystallinity(Raman intensity).

FIG. 42 is a schematic view showing a state in which a crystallizationstep is being performed with the use of an excimer laser.

FIG. 43 illustrates an apparatus for the experiment for studying therelationships between ambient pressures and degrees of crystallinity inlaser annealing.

FIGS. 44(a) and 44(b) are schematic views illustrating a method ofproducing a polysilicon thin film of Example 5-1.

FIG. 45 is a graph showing the result of measuring Raman scattering inthe polysilicon thin films of Examples 5-1 and 5-2.

FIG. 46 is a schematic view illustrating a method of producing apolysilicon thin film of Example 5-2.

FIG. 47 is a graph showing a transmissivity characteristic of glass.

FIG. 48 is a perspective view showing a configuration of a glasssubstrate on which a microcrystalline silicon thin film of Example 5-3is formed.

FIG. 49 is a schematic view illustrating a method of producing apolysilicon thin film of Example 5-3.

FIG. 50 is a graph showing characteristics of TFTs of Examples 5-3 to5-9.

FIG. 51 is a schematic view illustrating another example of the methodof producing a polysilicon thin film of Example 5-3.

FIG. 52 is a schematic view illustrating a method of producing apolysilicon thin film of Example 5-4.

FIG. 53 is a schematic view illustrating a method of producing apolysilicon thin film of Example 5-5.

FIG. 54 is a schematic view illustrating methods of producing apolysilicon thin film of Examples 5-6 and 5-7.

FIG. 55 is a graph showing the relationship between heating temperaturesand crystal grain diameters in Example 5-7.

FIG. 56 is a graph showing the relationship between heating temperaturesand field effect mobilities in Example 5-7.

FIG. 57 is a schematic view illustrating a method of producing apolysilicon thin film of Example 5-8.

FIG. 58 illustrates timings of irradiation in Example 5-8.

FIG. 59 is a schematic view illustrating a method of producing apolysilicon thin film of Example 5-9.

FIG. 60 illustrates a region in a liquid crystal display to beirradiated with a laser light in Example 6-1.

FIG. 61 a schematic view illustrating a method of applying a laser lightin Example 6-1.

FIGS. 62(a) and 62(b) are schematic views showing a laser annealingapparatus in Example 6-2.

FIG. 63 illustrates a region to be irradiated with a laser light inExamples 6-2 and 6-3.

FIG. 64 is a graph showing a dependency of mobilities on numbers oflaser irradiation times in Example 6-3.

FIG. 65 is a schematic view showing a laser annealing apparatus inExample 6-5.

FIG. 66 is a plan view showing a configuration of a mask material inExample 6-5.

FIG. 67 is a schematic view illustrating a method of laser annealing ofExample 6-6.

FIG. 68 is a schematic view illustrating another example of a method oflaser annealing of Example 6-6.

BEST MODE FOR CARRYING OUT THE INVENTION

Now, with reference to the figures, preferred embodiments of the presentinvention are detailed below.

EXAMPLE 1-1

Referring now to FIGS. 12(a) to 12( c), detailed below is an example inwhich crystal growth is controlled by providing on a substrate aplurality of regions each having a different thermal conductivity andthereby allowing a semiconductor thin film to have a temperaturedistribution.

As shown in FIG. 12(c), a lower insulating film 202 is formed on theentire region of a transparent insulating substrate 201 composed of, forexample, a glass substrate. On a part of the lower insulating film 202,a stripe-like shaped upper insulating film 203 is formed. The upperinsulating film 203 is composed of a material having a smaller thermalconductivity than the lower insulating film 203. On the lower insulatingfilm 202 and the upper insulating film 203, an amorphous silicon thinfilm 204 is formed.

By irradiating the amorphous silicon thin film 204 with a line-likelaser light having an energy density distribution in the x and ydirections as shown in FIG. 12(a), a polycrystalline silicon thin film210 is formed. In the formation of the polycrystalline silicon thin film210, since the thermal conductivity of the upper insulating film 203 islower than that of the lower insulating film 202, the temperature of aregion in the amorphous silicon thin film 204 over the upper insulatingfilm 203 becomes higher than the temperature of a region in theamorphous silicon thin film 204 over an interval of the upper insulatingfilms 203, as shown in FIG. 12(b). Therefore, the crystallization of theamorphous silicon thin film 204 starts from the region over the intervalof the upper insulating films 203, and the crystal growth proceedstowards the regions over the upper insulating films 203. As a result, inthe region over the interval of the upper insulating films 203, acollision of the crystal grains each other rarely occurs, and a largecrystal grain region 210 b in which crystal grains are relatively largeis thereby formed. On the other hand, in the region over the upperinsulating film 203, each of the crystal grains grown from both sides ofthe upper insulating films 203 collides with each other, andconsequently a small crystal grain region 210 a is formed.

In order to compare the degree of crystallinity, a polycrystallinesilicon thin film 210 thus formed and a prior art polycrystallinesilicon thin film polycrystallized in accordance with a prior art methodwere both subjected to a Raman spectroscopy, the peak strengths of Ramanspectrum were compared. The results are shown in FIG. 13.

For this measurement, the polycrystalline silicon thin film 210 of thepresent example employs a silicon nitride thin film having a thicknessof 200 nm (thermal conductivity: 0.19 W/cm·° C.) as the lower insulatingfilm 202 and a silicon oxide thin film having a thickness of 30 nm and awidth of approx. 5 μm (thermal conductivity: 0.014 W/cm·° C.) as theupper insulating film 203 with an interval between each of the upperinsulating films 203 being 20 μm. On the other, the prior artpolycrystalllne silicon thin film employs a single layer of a siliconoxide thin film having a thickness of 200 nm. In each of thepolycrystalline silicon thin films, the thickness of the amorphoussilicon thin film was made to be 85 nm. In each of the films, themeasuring points of the Raman peak strengths were the central region ofthe region to be irradiated along an x direction in FIGS. 12(a) to12(c).

As apparent from FIG. 13, in the case of the polycrystalline siliconthin film made by a prior art method, the degree of crystallinity isrelatively small over the entire region. By contrast, in the case of themethod according to the present invention, whereas the degree ofcrystallinity is small in the regions over the upper insulating film203, which are denoted by A, B, and C in the figure, the regions overthe lower insulating film 202 sandwiched by the upper insulating films203 show large Raman peak strengths. This indicates that the degree ofcrystallinity is remarkably improved by the method according to thepresent invention.

It is noted that the optimal value for the interval between each of thestripe-like shaped upper insulating films 203 varies depending uponthermal conductivities of the lower insulating film 202 and the upperinsulating film 203, energy densities of the light to be irradiated, andso forth. However, in the example described above, it is preferable tomake the interval to be within the range of 5-50 μm, more preferablywithin the range of 10-30 μm, in order to reliably obtain largecrystals.

The description above shows an example in which the surface of thesilicon thin film has a temperature distribution with respect to the ydirection in FIGS. 12(a) to 12(c). However, in the case where a laserbeam is applied in a stationary state, the temperature distribution maybe also given with respect to the x direction in like manner. In thecase of the laser beam applied by scanning in the x direction, theinfluence by the sequential movement of the region to be irradiated onthe temperature distribution should be taken into consideration. Inaddition to utilizing the difference of thermal conductivities asdescribed above, an adjustment of the temperature distribution may beperformed by varying an energy density distribution in each of theregions.

In the example described above, the thermal conductivity of the upperinsulating film 203 is made smaller than the thermal conductivity of thelower insulating film 202, so as to increase the grain size in theregions under which the upper insulating film 203 is absent. Conversely,the thermal conductivity of the upper insulating film may be made largerthan the thermal conductivity of the lower insulating film, thereby toincrease the grain size in the regions under which the upper insulatingfilm is present. Nevertheless, the former method can more readilyincrease an area of the region in which the thermal conductivity ishigh, i.e., the surface temperature of the silicon thin film is low,therefore can more readily increase the temperature gradient in thetemperature distribution on the surface of the silicon.

The relationship between a stacking order and thermal conductivities ofthe insulating films is not limited to the manner described above, butmay be the reverse order, insofar as a predetermined temperaturedistribution is attained.

In the case where the insulating film has a two-layer structure asdescribed above, the upper insulating film can be easily formed into adesired shape (thickness) by setting an etching selective ratio (a ratioof etching rates) between the upper insulating film and the lowerinsulating film. Therefore, the upper insulating film can be made tohave a uniform thickness over a wide area, and as a result, apolycrystalline silicon thin film having a uniform grain size in theentire surface of the substrate can be readily obtained.

In order to provide regions each having a different thermalconductivity, a thickness of the silicon thin film may be varied byetching or the like method. By employing this method, althoughrelatively high accuracy is needed in the etching, the manufacturingsteps can be simplified since forming two layers of insulating filmsbecomes unnecessary.

In place of varying the thermal conductivities, the temperaturedistribution can be provided by forming a plurality of regions eachhaving a different thermal capacity. This can also improve crystallinityof the silicon thin film.

EXAMPLE 1-2

Example 1-2 describes an example of a polycrystalline silicon thin filmtransistor employing the semiconductor thin film made in accordance withthe foregoing example.

FIG. 14(a) shows a plan view of the polycrystalline silicon thin filmtransistor, and FIG. 14(b) shows a cross section taken along the lineA-A′ in FIG. 14(a). In FIG. 14(a), there are shown a transparentinsulating substrate 201, a lower insulating film 202, a upperinsulating film 203, a gate insulating film 205, a source electrode film206, a drain electrode film 207, a gate electrode film 208, and a largecrystal grain region 210 b in a polycrystalline silicon thin film 210.The polycrystalline silicon thin film 210 is polycrystallized in themanner described in the above Example 1-1. In the polycrystallinesilicon thin film transistor of the present example, only the largecrystal grain region 210 b sandwiched by the upper insulating films 203is selectively left unremoved by etching or the like method, and asource-drain direction is made to be in parallel with a direction of astripe-like pattern formed by the upper insulation films 203.

The gate insulating film 205, the source electrode film 206, the drainelectrode film 207, and the gate electrode film 208 can be formed by thesame methods as used for forming conventional thin film transistors,such as a thin film deposition and a patterning.

The polycrystalline thin film transistor thus obtained exhibited a fieldeffect mobility of approximately 180 cm²/V·sec, whereas a transistormade in accordance with a prior art method showed a field effectmobility of 70 cm²/V·sec, which demonstrates that a remarkableimprovement in TFT characteristics was achieved.

It is to be noted that the relationship between the direction of theupper insulating film 203 and the source-drain direction is not limitedto the above-described relationship, in which both are directed to thesame direction. Many variations are possible insofar as the source-draindirection is made to be in the lengthwise direction of the crystalgrains formed according to the interval between each of the upperinsulating films 203 and so forth.

EXAMPLE 1-3

Example 1-3 describes an example in which a polycrystalline silicon thinfilm transistor having a larger size than the transistor produced inaccordance with the foregoing Example 1-2.

A polycrystailine silicon thin film transistor of Example 1-3 differsfrom the thin film transistor of the foregoing Example 1-2 in that twolarge crystal grain regions 210 b formed between three upper insulatingfilms 203 are employed to form the transistor. In addition, a siliconoxynitride thin film formed by a plasma CVD having a thickness ofapproximately 200 nm is used for the lower insulating film 202, and asilicon oxide thin film having a thickness of approximately 40 nm isused for the upper insulating films 203. Each of the large crystal grainregions 210 b is formed by firstly forming an amorphous silicon thinfilm 204 having a thickness of 85 nm on the upper insulating film 203formed by pattering, and then irradiating the amorphous silicon thinfilm 204 with an excimer laser as in Example 1-1 to obtain apolycrystalline silicon thin film.

When the interval of the upper insulating films 203 is increased inorder to increase the size of the transistor to be produced, it becomesdifficult in the polycrystallization treatment to form a sufficienttemperature gradient between the region in which a transistor is to beformed and the peripheral regions sandwiching the region in which atransistor is to be formed, and as a result, there is a possibility thatthe size of crystal grains in the region in which a transistor is to beformed cannot be made sufficiently large. In view of this problem, thetemperature gradient is deliberately increased instead of increasing theinterval of the upper insulating films 203, and a plurality of largecrystal grain regions 210 b with a desirable crystal condition areformed. By combining these, a large-sized thin film transistor withdesirable characteristics can be achieved. Specifically, for example, atransistor made in accordance with the present example exhibited a fieldeffect mobility of approximately 200 cm²/V·sec, which was a remarkablygood characteristic.

As has been described thus far, according to a method for producing apolycrystalline silicon thin film transistor of the present invention,it is made possible to make large crystal grains exclusively in a regionin which a transistor is to be formed. It is, however, to be noted thata material for the insulating film to be formed on the transparentinsulating substrate is not limited to silicon nitride, siliconoxynitride, and silicon oxide. Other materials may be employed insofaras the materials have different thermal conductivities from each otherand a selective etching for the materials is possible.

Example 2-1

As a semiconductor device of Example 2-1, an example of a thin filmtransistor as a semiconductor device having large crystal grains isdescribed below.

FIGS. 16(a) and 16(b) show schematic views of a thin film transistor.FIG. 16(a) shows a plan view thereof, and FIG. 16(b) shows a crosssectional view taken along the line A-A′ in FIG. 16(a).

Referring to FIGS. 16(a) and 16(b), an insulating substrate 301 isshown. On the insulating substrate 301, an undercoat layer 302 isprovided, and on the undercoat layer 302, a semiconductor layer 303 isformed by crystallizing an amorphous semiconductor film. In thesemiconductor layer 303, a plurality of protruding parts 303 a areformed with predetermined intervals, and each of the protruding parts303 a extends outwardly in the same plane as that of the semiconductorlayer 303. Each protruding part 303 a is so formed as to have anapproximately rectangular shape, and the length (a length of a lineprotruding from the semiconductor layer 303) and the width (a length ofa line perpendicular to the line protruding from the semiconductor layer303) are made to be 1 μm. A first insulating layer 304 is provided overthe semiconductor layer 303 so as to cover the semiconductor layer 303,and a gate electrode 305 as a first electrode is provided at apredetermined position on the first insulating layer 304. A secondinsulating layer 306 is provided so as to cover the gate electrode 305,and a source electrode 307 s and a drain electrode 307 d are provided ata predetermined position on the second insulating layer 306, as a pairof second electrodes electrically connected with the semiconductor layer303.

It is noted here that the width of the protruding part 303 a is notlimited to 1 μm. However, in order to provide one crystal nucleus foreach of the protruding parts 303 a, it is preferable to make the widthwithin the range of from a width approximately equal to or greater thanthe thickness of the semiconductor layer 303 (for example, 0.05 μm) toapproximately 3 μm. The technical reason why the above range ispreferred is as follows: on one hand, when the width of the protrudingpart 303 a is smaller than the thickness of the semiconductor layer,there is a possibility that the crystal nucleus to be grown in theprotruding part 303 a is dragged into the semiconductor layer 303 by theeffect of surface tension, and therefore the crystal nucleus cannotcontinue to exist; on the other hand, when the width of the protrudingpart 303 a is larger than 3 μm, there is a possibility that two or morecrystal nuclei are grown on the protruding part 303 a. It is noted thatthe shape of the protruding part 303 a may be other shapes than arectangular shape, such as a semicircular shape and a triangular shape.In addition, the plurality of the protruding parts 303 a need not beformed along the entire lengthwise line segments opposed to each otherin the semiconductor layer 303, and may be provided, for example, onlyin the region corresponding to the gate electrode 305. That is, theprotruding parts 303 a should be provided at least in a channel regionthat affects the characteristics of the device. In addition, theprotruding parts 303 a may be so formed that they are disposed in thevicinity of an intermediate position between the source and the drain.An interval between each of the protruding parts 303 a next to eachother may be suitably selected depending upon such conditions as adesired grain size and so forth. In the present example, the intervalbetween each of the protruding parts 303 a is set to be approximatelyequal to the length (W) of a line segment perpendicular to the linesegment along which the protruding parts 303 a are provided. Suchsetting is preferable in that by such setting, a large crystal grainthat has approximately the same lengths in both lengthwise and widthwisedirections is likely to be formed. However, even if such setting is notemployed, the advantageous effect that relatively large crystal grainsare formed can be obtained since crystals are grown from the peripheralregions in a controlled manner.

Because of the provision of such protruding parts 303 a in thesemiconductor layer 303, the protruding parts 303 a are cooled earlierafter the semiconductor layer 303 is heated by the laser beamirradiation, which makes the formation of crystal nuclei easy to takeplace therein, and thereafter, crystals are grown from the formedcrystal nuclei towards a central region of the semiconductor layer 303.During this process, the crystal grains grown from the adjacentprotruding parts 303 a next to each other and from the opposingprotruding parts 303 a tend to grow until they reach the vicinity of acentral region of the semiconductor layer 303 without interfering eachother, and consequently, relatively large crystal grains are formed.Hence, the field effect mobility is increased and TFT characteristicsare improved.

Now, with reference to FIGS. 18(a) to 18(e), a method of producing theabove-described thin film transistor is detailed below. FIGS. 18(a) to18(e) illustrate the steps of producing such a thin film transistordescribed above.

Firstly, as shown in FIG. 18(a), an undercoat layer 302 is formed on aninsulating substrate 301, and silicon is attached on the undercoat layer302 to form an amorphous (non-single crystal) semiconductor layer 303.Secondly, a photoresist (not shown) is selectively formed in apredetermined shape, and using the photoresist as a mask, the amorphoussemiconductor layer 303 is formed into such a shape as shown in theforegoing FIG. 16(a) that a plurality of the protruding parts 303 aextending outwardly in the same plane as the amorphous semiconductorlayer 303 are provided along the whole lengths of the opposed linesegments of the amorphous semiconductor layer 303. Thereafter, thephotoresist is removed.

Subsequently, as shown in FIG. 18(b), the amorphous semiconductor layer303 is irradiated with an excimer laser as an energy beam to crystallizethe amorphous semiconductor layer 303 into a modified layer of poly-Si.Here, after the laser light irradiation, the heat accumulated in aprotruding part 303 a situated in the peripheral region can diffuse inthree outward directions in a plane parallel to the semiconductor layer303, whereas the heat accumulated in the central region cannot diffuseexcept towards the peripheral region that is not yet cooled. Therefore,the peripheral region including the protruding part 303 a is cooledsufficiently earlier than the central region. As a result, a crystalnucleus in the protruding part 303 a is formed in an earlier stage thancrystal nuclei in the central region, and the crystal nucleus formed inthe peripheral region grows towards the central region before crystalnuclei are formed or grown in the central region. Thereby, it is madepossible to control a crystal grain size and a crystal orientation.Thus, the interference between crystals during the process of crystalgrowth is prevented, and a sufficient crystal grain size can be easilyobtained.

Subsequent to the above, as shown in FIG. 18(c), a first insulatinglayer 304 is formed on the semiconductor layer 303 and the undercoatlayer 302, and a gate electrode 305 as a first electrode, is selectivelyformed on the first insulating layer 304.

Thereafter, as shown in FIG. 18(d), utilizing the gate electrode 305 asa mask, a source region 303 s and a drain region 303 d are formed byadding an impurity serving as a donor or an acceptor into thesemiconductor layer 303 with the use of an ion implantation method or anon-mass separation ion doping method.

Finally, as shown in FIG. 18(e), a second insulating layer 306 isformed, and thereafter contact holes are opened and a source electrode307 s and a drain electrode 307 d are selectively formed, therebycompleting a thin film transistor.

Although Si is used for the semiconductor layer 303 in the presentexample, other materials such as a compound of Si and Ge may also beemployed. Other combinations of group IV elements such as SiC,combinations of a group III element and a group V element such as GaAs,combinations of a group II element and a group VI element such as CdSeare also possible. In addition, although an example of a polycrystallinesilicon thin film transistor has been described here, the presentinvention is not limited thereto and can be suitably applied to othervarious semiconductor devices.

Further, although an excimer laser is used as an energy beam topolycrystailize the amorphous semiconductor layer 303 in the presentexample, other energy beams may be employed. Examples of other energybeams include a laser light such as Ar laser, YAG laser, and so forth,ion beams, and electron beams.

Example 2-2

Now, an example of an inverted staggered type thin film transistor isdetailed as a semiconductor device of Example 2-2.

FIGS. 17(a) and 17(b) schematically show the thin film transistoraccording to Example 2-2. FIG. 17(a) shows a plan view of the thin filmtransistor, and FIG. 17(b) shows a cross-sectional view taken along theline A-A′ in FIG. 17(a).

The thin film transistor of Example 2-2 differs from the thin filmtransistor of the foregoing Example 2-1 primarily in that the transistorhas an inverted staggered structure and that the protruding parts 303 aare formed along the whole perimeter line of the semiconductor layer303.

Referring now to FIGS. 17(a) and 17(b), an insulating substrate 301 isshown. On the insulating substrate 301, an undercoat layer 302 isprovided, and on the undercoat layer 302, a gate electrode 305 isprovided as a first electrode. A first insulating layer 304 is providedover the gate electrode 305, and on the first insulating layer 304, asemiconductor layer 303 is provided. This semiconductor layer 303 has,as shown in FIG. 17(a), a plurality of protruding parts 303 a extendingoutwardly in the same plane as the semiconductor layer 303, and theprotruding parts 303 a are formed along the whole perimeter line of thesemiconductor layer 303 so as to have a predetermined interval betweeneach of the protruding parts. The shape and other conditions of eachprotruding part 303 a are the same as those in the foregoing Example2-1. It is preferred that each of the intervals between the protrudingparts 303 a is approximately equal to the width of the semiconductorlayer 303 as in the foregoing Example 2-1, although in FIG. 17(a), theintervals are drawn to be narrower for the sake of convenience in thedrawing. It is noted, however, that even if the interval is made to benarrower as drawn in FIG. 17(a) or made to be wider, the advantageouseffect that relatively large crystal grains are formed can be obtainedsince crystals are grown from the peripheral regions in a controlledmanner. On the semiconductor layer 303, a source electrode 307 s and adrain electrode 307 d are formed, as a pair of second electrodeselectrically connected with the semiconductor layer 303.

Now, with reference to FIGS. 19(a) to 19(e), a producing method of theabove-described thin film transistor is described. FIGS. 19(a) to 19(e)illustrate the steps of producing the thin film transistor.

Firstly, as shown in FIG. 19(a), an undercoat layer 302 is formed on ainsulating substrate 301, and a gate electrode 305 as a first electrodeis selectively formed on the undercoat layer 302.

Secondly, as shown in FIG. 19(b), a first insulating layer 304 is formedover the gate electrode 305 and the undercoat layer 302, and anamorphous (non-single crystalline) semiconductor layer 303 is formed bycoating silicon on the first insulating layer 304. Then, a photoresist(not shown) is selectively formed on the semiconductor layer 303 into apredetermined shape, and using the photoresist as a mask, the amorphoussemiconductor layer 303 is formed in such a shape that a plurality ofprotruding parts 303 a extending outwardly in the same plane as thesemiconductor layer 303 are provided along the whole perimeter line, asshown in the foregoing FIG. 17(a). Thereafter the photoresist isremoved.

Subsequently, as shown in FIG. 19(c), the amorphous semiconductor layer303 is irradiated with an excimer laser light as an energy beam tocrystallize the amorphous semiconductor layer 303 into a modified layerof poly-Si. In this stage, because the protruding parts 303 a are formedas described above, a sufficient crystal grain size can be readilyachieved for the reasons described in the foregoing Example 2-1.

Thereafter, as shown in FIG. 19(d), a resist 308 serving as a maskagainst doping is selectively formed on the semiconductor layer 303 tohave a predetermined shape, and utilizing the resist 308 as a mask, asource region 303 s and a drain region 303 d are formed by adding animpurity serving as a donor or an acceptor into the semiconductor layer303 with the use of an ion implantation method or a non-mass separationion doping method. Then, the resist 308 is removed.

Finally, as shown in FIG. 19(e), a source electrode 307 s and a drainelectrode 307 d are selectively formed, and thus a thin film transistoris obtained.

It is to be noted that various modifications described in the foregoingExample 2-1 are also possible in Example 2-2.

In addition, the present example is not limited to the application to aninverted staggered type thin film transistor as described, and the sameadvantageous effects as those in the present example can be alsoobtained when a staggered type thin film transistor as in the foregoingExample 2-1 is produced accordingly. Further, instead of forming theprotruding parts 303 a along the whole perimeter line of thesemiconductor layer 303 as described above, the protruding parts 303 amay be formed only along the opposed lines as described in the foregoingExample 2-1.

Example 3-1

Example 3-1 is now detailed with reference to FIGS. 20-22. A structureof a thin film transistor (TFT) according to Example 3-1 is detailedfirst.

FIGS. 20(a) and 20(b) schematically show the structure of a staggeredtype TFT 410. FIG. 20(a) shows a plan view of the TFT 410, and FIG.20(b) shows a cross-sectional view taken along the line A-A′ in FIG.20(a) FIG. 21 shows a cross-sectional view taken along the line B-B′ inFIG. 20(a). As shown in FIGS. 20(a) and 20(b), the TFT 410 comprises aninsulating substrate 401, and on the insulating substrate 401, there areprovided an undercoat layer 402, a p-Si film 403, a first insulatingfilm 404, a second insulating film 406, a gate electrode 405, and threeelectrodes, namely a gate electrode 405, a source electrode 407 s, and adrain electrode 407 d.

The insulating substrate 401 is composed of, for example, a glasssubstrate having a thickness of 1.1 mm and a glass transitiontemperature of 593° C., and the undercoat layer 402 is composed of, forexample, a thin film comprising SiO₂. The p-Si film 403 is apolycrystalline semiconductor layer, which is formed on the undercoatlayer 402 by using the method of the present invention. The p-Si film403 has a channel region 403 a, a source region 403 b, and a drainregion 403 c, and the channel region 403 a is, disposed between thesource region 403 b and the drain region 403 c. The source region 403 band the drain region 403 c are produced by doping impurity ions such asphosphorus ions, boron ions, and so forth.

Regarding the material for the p-Si film 403, silicon (Si), or acompound of silicon and germanium (Ge) may be employed. A thickness ofthe p-Si film 403 is preferable to be within the range of 200 Å-1500 Å,or more preferably within the range of 300 Å-1000 Å. If the thickness isless than 200 Å, a uniform film thickness is difficult to obtain. On theother hand, if the thickness is greater than 1500 Å, a problem ofso-called photoconduction is caused, the problem in which an electriccurrent flows between the source and drain by the light irradiation.However, when the thickness is within the range of 300 Å-1000 Å, both auniform thickness and the suppression of photoconduction are achieved.

In addition, a width of the channel region 403 a in a direction of thearrow X in FIG. 20(a) is, for example, approximately 12 μm, and a widthof the p-Si film 403 in a direction of the arrow Y is, for example,approximately 14 μm.

In the channel region 403 a, as shown in FIG. 20(a) and FIG. 21, aplurality of groove-like gaps 411 for controlling an orientation ofcrystal growth are formed so as to be parallel to a line linking thesource region 403 b and the drain region 403 c. Each of the gaps 411 hasa semicircular edge at both lengthwise edges and a middle portion havinga rectangular parallelepiped shape, and a width of the groove in themiddle portion (a width of the groove in a direction perpendicular tothe lengthwise direction) is approximately 1 μm. It is noted, however,that the shape of the gaps 411 are not particularly restricted. Forexample, the gap 411 may be formed into a rectangle or the like shape,and made to be parallel to a line linking the source region 403 b andthe drain region 403 c.

In the channel region 403 a, each of the crystal grains results in ashape extending longitudinally and narrowly towards the source region403 b or the drain region 403 c, and a multiplicity of such crystalgrains constitute a polycrystalline semiconductor layer of the channelregion 403 a. In the channel region 403 a having such a polycrystallinestructure, a density of grain boundaries is small along the direction ofa line connecting the source region 403 b and the drain region 403 c,and therefore charge carriers can be transferred at high speed.

The first insulating film 404 is composed of, for example, SiO₂, and isformed over the p-Si film 403 and the undercoat layer 402. The gateelectrode 405 is composed of, for example, aluminum (Al) and the like,and is provided on the first insulating film 404 and at a positioncorresponding to the channel region 403 a of the p-Si film 403. Thesecond insulating film 406 is composed of, for example, SiO₂, and isstacked over the first insulating film 404 and the gate electrode 405.

In the first insulating film 404 and the second insulating film 406,contact holes 408 are formed, and each of the contact holes 408 reachesthe source region 403 b or the drain region 403 c of the p-Si film 403.The source electrode 407 s and the drain electrode 407 d are composedof, for example, Al, and are so formed to be in contact with the sourceregion 403 b or the drain region 403 c via the contact hole 408. Thewiring pattern of the gate electrode 405, the source electrode 407 s,and the drain electrode 407 d is constructed by pattering into apredetermined shape in a position not the cross sectional surface shownin FIG. 21.

Now, with reference to the figures, a method of producing the TFT 410according to Example 3-1 is detailed below.

FIGS. 22(a) to 22(e) schematically show cross sectional views forillustrating steps of producing the TFT 410. Firstly, as shown in FIG.22(a), the undercoat layer 402 is formed on the insulating substrate 401by an atmospheric pressure CVD method. A thickness of the undercoatlayer 402 is, for example, 3000 Å.

On the undercoat layer 402, an Si layer is formed by, for example, aplasma CVD method, and on the Si layer, a photoresist (not shown) isselectively formed to have a predetermined shape. Then, using thephotoresist as a mask, an exposure is carried out, and thereafterpatterning is performed by etching to obtain a predetermined pattern.Thereafter, the photoresist is removed.

Thus, an —Si film 413 is formed as a non-single crystallinesemiconductor layer having a plurality of gaps 411 for controlling anorientation of crystal growth. The thickness of the a-Si film 413 ismade, for example, 650 Å. In the case where the gaps 411 need to beformed in a very small size, a high accuracy photoresist and an exposureby an interference pattern of coherent lights can serve the purpose.

Subsequent to the formation of the a-Si film 413, the a-Si film 413 isheated by irradiating the entire surface of the a-Si film 413 with oneshot of an excimer laser as shown in FIG. 22(b), and thereafter cooled.The p-Si film 403 as a crystalline semiconductor layer is therebyformed.

By employing a crystallization method utilizing an excimer laser, thetemperature of the main portion of the a-Si film 413 is sufficientlyraised since the a-Si film 413 has a large absorption coefficient in arange of ultraviolet rays, whereas the region of the gaps 411, fromwhich a-Si is removed, is maintained at a low temperature because thelaser light is not absorbed in the region of the gaps 411. As a result,in the process of cooling, the temperature in the vicinity of the gaps411 (and the peripheral region of the a-Si film 413) first reaches thecrystallization-starting temperature, and crystal nuclei are initiallyformed in this region. Thereafter, the crystal growth continues to takeplace from this crystal nucleus. As has been mentioned above, theorientation of crystal growth is restricted by the gaps 411 eachprovided in parallel, and thereby is guided in a direction parallel tothe direction linking the source region 403 b and the drain region 403c. Hence, the resulting p-Si film has a small density of grainboundaries in the direction parallel to the direction linking the sourceregion 403 b and the drain region 403 c.

Regarding the conditions of the irradiation of the energy beam, forexample, in the case of an excimer laser of XeCl (wavelength: 308 nm)and the like, a laser light pulse of 50 ns in which a cross-sectionalshape of the beam is a rectangle having each line segment of severalmillimeters, is used. The energy density (irradiating energy per unitarea: mJ/cm²) may be selected appropriately so that the a-Si film 413can be heated to a temperature suitable for the crystallization.

For the excimer laser, other excimer lasers of such as ArF, KrF, XeF andthe like may be used other than XeCl as above. An interval between theplurality of gaps 411 may be appropriately selected depending upon athickness of the a-Si film, conditions of the laser irradiation, and adesired charge carrier mobility. In this example, the interval is set atapproximately 2 μm. A width of the gaps 411 may also be appropriatelyselected depending upon a thickness of the a-Si film, a type andintensity of the energy beam to be irradiated, and the like. In Example3-1, the width is made to be approximately 1 μm.

After the crystallization of the a-Si film as described above, the firstinsulating film 404 is formed on the p-Si film 403 as shown in FIG.22(c) by an atmospheric pressure CVD method, so as to have a thicknessof 1000 Å. Then, by forming an Al film on the first insulating film 404so as to have a thickness of 2000 Å with the use of sputtering, and thencarrying out a wet etching for approximately 1 minute with the use of anAl etchant liquid, the gate electrode 405 and a predetermined wiringpattern are formed.

Subsequently, as shown in FIG. 22(d), using the gate electrode 405 as amask, impurity ions serving as donors or acceptors, such as,specifically, phosphorus ions and boron ions, are implanted in the p-Sifilm 403 by an ion implantation method or a non-mass separation iondoping method. The channel region 403 a, the source region 403 b, andthe drain region 403 c are thus formed in the p-Si film 403.

Then, as shown in FIG. 22(e), the second insulating film 406, composedof, for example, SiO₂, is formed on the gate electrode 405 by anatmospheric pressure CVD method, so as to have a thickness of 5000 Å.Thereafter, the contact holes 408 are formed in the first insulatingfilm 404 and the second insulating film 406 so as to reach the sourceregion 403 b or the drain region 403 c in the p-Si film 403. Followingthis, an Al film is formed by sputtering to have a thickness of 3000 Å,and a predetermined pattern is formed by dry etching using a BC13/C12type gas. The source electrode 407 s and the drain electrode 407 d, andthe wiring pattern of these are thereby formed.

According to Example 3-1 as described thus far, it is made possible toform large crystal grains longitudinally extending in the directionlinking the source region 403 b and the drain region 403 c, and as aresult, a staggered type TFT having an excellent field effect mobilityis achieved. In addition, the present example does not employ costlymaterials for the insulating substrate 401 and the p-Si film 403 andthus can provide TFTs having an excellent field effect mobility at lowcost.

Example 3-2

Example 3-2 in accordance with the present invention is detailed withreference to FIGS. 23 to 25. Of the components of a thin film transistorin accordance with Example 3-2, the components having like functionscorresponding to Example 3-1 are represented by like reference numbersand characters, and such components will not be further elaborated uponhere.

FIGS. 23(a) and 23(b) show schematic views of an inverted staggered typeTFT 420 in accordance with Example 3-2, wherein FIG. 23(a) shows a planview of the TFT 420, FIG. 23(b) shows a cross sectional view taken alongthe line A-A′ in FIG. 23(a). FIG. 24 shows a cross sectional view takenalong the line B-B′ in FIG. 23(a). As shown in FIGS. 23(a) and 23(b),the TFT 420 is provided with, on an insulating substrate 401, anundercoat layer 402, a p-Si film 403, a first insulating film 404, andthree electrodes, namely, a gate electrode 405, a source electrode 407 sand a drain electrode 407 d.

The gate electrode 405 is formed on the undercoat layer 402 formed overthe insulating substrate 401. The first insulating film 404 is formedover the undercoat layer 402 and the gate electrode 405. Further, thep-Si film 403 is formed over the first insulating film 404.

In a channel region 403 a in the p-Si film 403, a plurality ofgroove-like gaps 411 are formed in a direction from a source region 403b towards a drain region 403 c as in the foregoing Example 3-1 (see FIG.23(a) and FIG. 24). The source electrode 407 s and the drain electrode407 d are formed so as to be in contact with the source region 403 b andthe drain region 403 c in the p-Si film 403 respectively. It is notedthat in a region not the cross sectional surfaces shown in the figures,the gate electrode 405, the source electrode 407 s and the drainelectrode 407 d are patterned into a predetermined shape to form awiring pattern.

Now, a method of producing the TFT 420 in accordance with the presentexample is detailed with reference to FIGS. 25(a) to 25(e). FIGS. 25(a)to 25(e) show schematic cross sectional views illustrating productionsteps of the TFT 420. First, in a manner analogous to that in theforegoing Example 3-1, the undercoat layer 402 is formed on theinsulating substrate 401. Subsequently, on the undercoat layer 402, thegate electrode 405 and the wiring pattern are formed by performing apatterning into predetermined shapes (see FIG. 25(a)).

Next, as shown in FIG. 25(b), the first insulating film 404 is formedover the gate electrode 405 and the undercoat layer 402. Further, in amanner analogous to that in Example 3-1, an Si layer is formed on thefirst insulating film 404 by for example a plasma CVD method. After aphotoresist is selectively formed on the Si layer in a predeterminedshape, an exposure is implemented using the photoresist as a mask, andthereafter a patterning is performed by etching to obtain apredetermined shape. Thereafter, the photoresist is removed. Thus, thea-Si film 413 having a plurality of gaps 411 for controlling anorientation of crystal growth is formed. Then, as shown in FIG. 25(c),an excimer laser is applied to the entire surface of the a-Si film 413to crystallize the a-Si film 413, and thus the p-Si film 403 is formed.

Since the gaps 411 for controlling are provided in the channel region403 a in the p-Si film 403, the formed crystal grains have a long andnarrow shape extending towards the source region 403 b or the drainregion 403 c. As a consequence, grain boundaries al direction of thestraight line linking the source region 403 b and the drain region 403 care practically reduced, and therefore the field effect mobility isimproved.

Then, as shown in FIG. 25(d), a resist is applied onto the p-Si film403, and patterned into a predetermined shape by an exposure anddevelopment to form a resist mask 414 as an ion blocking film. Theresist mask 414 is not particularly limited insofar as the material canblock impurity ions, and various known materials may be employed.Specifically, for example, a positive resist such as OFPR-5000(trademark, manufactured by Tokyo Ohka Kogyo Co., Ltd.) may be employed.In addition, the resist mask 414 is not limited to materials havingphotosensitivity such as the resist, but may be made of such materialsthat patterning by photolithography is possible.

Using the resist mask 414 as a mask, impurity ions of such as phosphorusions and boron ions are implanted into the p-Si film 403 by using, forexample, an ion doping method. Thereby, the channel region 403 a isformed on the p-Si film 403 and the source region 403 b, and the drainregion 403 c are formed on the respective sides of the channel region403 a. Thereafter, the resist mask 414 is removed, and further, as shownin FIG. 25(e), the source electrode 407 s and the drain electrode 407 dare selectively formed. Thus, an inverted staggered type TFT 420 inaccordance with Example 3-2 is produced.

The inverted staggered type TFT thus produced also achieves theadvantageous effects as obtained in the foregoing Example 3-1, such asan improvement in the field effect mobility and so forth.

Example 3-3

Example 3-3 is characterized in that an early-crystallization region inwhich crystallization starts at a higher temperature than in otherregions is provided, in place of the gaps for controlling an orientationof crystal growth employed in the foregoing Examples 3-1 and 3-2.Referring now to FIG. 26, a crystalline thin film semiconductortransistor in accordance with Example 3-3 is detailed below. Since thepresent example is identical to the foregoing Example 3-1 except thatthe early-crystallization region is provided in place of the gaps forcontrolling an orientation of crystal growth, no further elaborationexcept the details regarding the early-crystallization region will begiven here. The components having like functions corresponding toExamples 3-1 and 3-2 are designated by like reference numerals andcharacters.

As shown in FIG. 26, belt-like shaped early-crystallization regions 421are formed in the channel region of the p-Si 403 so as to extend fromthe source region towards the drain region. In the early-crystallizationregions 421, impurity ions other than phosphorus ions, boron ions, orthe like, are implanted. A TFT 430 having such a configuration isproduced in the following manner.

First, in a manner analogous to that in the foregoing Example 3-1, theundercoat layer 402 is deposited on the insulating substrate 401 byusing an atmospheric pressure CVD method. Thereafter, an Si layer isformed on the undercoat layer 402 by using, for example, a plasma CVDmethod, and on the Si layer, a photoresist is selectively formed in apredetermined shape. After the photoresist is exposed using thephotoresist as a mask, and then patterned into a predetermined shape byetching, and thus an a-Si film 413 is formed.

Subsequently, in the channel region 403 a in the a-Si film 413, impurityions capable of raising the crystallization-starting temperature, butnot phosphorus ions or boron ions, are implanted in belt-like regionsextending from the source region 403 b towards the drain region 403 c toform early-crystallization regions 421. Then, an excimer laser beam asan energy beam is applied to the entire surface of the a-Si film 413 inwhich the early crystallization regions 421 are already formed forapproximately 50 ns, and thereafter the a-Si film 413 was cooled for thecrystallization of the a-Si film 413.

When the energy beam is applied to the entire surface of the a-Si film413, the temperature of the surface of the a-Si film increases, and thenthe temperature gradually decreases by heat dissipation. In the processof the temperature decrease, crystal nuclei are first formed in theearly-crystallization region 421 earlier than in other regions. Thereason is that by implanting impurity ions therein, theearly-crystallization region 421 is formed so that crystallization takesplace at a higher temperature than in the other regions.

Thereafter, crystal growth takes place from the crystal nuclei formed inthe early-crystallization region 421. Thereby, a poly-Si film in whichlarge crystal grains are collectively grown can be formed.

It is noted that the fabrication steps after the crystallization are thesame as those in the foregoing Example 3-1.

The technique for implanting the impurity ions capable of raising thetemperature of starting the crystallization are not particularlylimited, and various known techniques may be employed. In addition,although the present example shows an example of a staggered type thinfilm transistor, the same advantageous effects can be obtained also ininverted staggered type transistors. The early-crystallization region isnot limited to the one as described above in which impurity ions areimplanted, but may be formed by such a manner that a crystallized region(pre-crystal) is partially formed in advance and the differences of themelting points corresponding to the degrees of crystallinity(crystallization temperatures) are utilized. In addition, in order toform such pre-crystals in a very small size, for example, theirradiation of interference patterns of coherent lights may be employed.

Supplementary Remarks for Example 3-1 to 3-3

The foregoing Examples 3-1 and 3-2 employ a plurality of groove-likegaps 411 for controlling an orientation of crystal growth longitudinallyextending in a direction linking the source region and the drain regionare provided in the a-Si film 413. However, the present invention is notlimited thereto. For example, a plurality of gaps 431 for controlling anorientation of crystal growth may be discontinuously provided in thedirection linking the source region and the drain region as shown inFIG. 27. In such a configuration, by appropriately adjusting theintervals of the gaps 431 in the direction linking the source region andthe drain region as well as the intervals of the adjacent gaps 431 in adirection perpendicular to the above-described direction, it is madepossible to control the size of the crystal grains in the foregoingdirection.

Further, in the present invention, it is possible to provide a pluralityof gaps having a depth such that the gaps do not pierce through the a-Sifilm as shown in FIG. 28. In addition, such gaps may be formed indiscontinuous island-like shapes.

When such gaps not piercing through the a-Si film are provided, theprotruding portions that form the gaps may be removed by etching or thelike after the step of forming the p-Si film, so as to planarize thesurface of the p-Si film.

In addition, for example, a member having a specific heat different fromthat of the main portion may be provided over the channel region of thea-Si film 413, and the member may have, for example, a rod-like shape.Further, a plurality of regions having different specific heats fromeach other may be provided for controlling an orientation of crystalgrowth. For example, if a member having a larger specific heat than thatof the a-Si film is placed thereon and an energy beam is applied for ashort time, the temperature increase in the portion of the a-Si film incontact with the member is small, and therefore crystal nuclei start toform earlier in the region than in other regions. On the other hand, forexample, if a plurality of members having a smaller specific heat thanthat of the a-Si film are placed over the a-Si film in the directionlinking the source region and the drain region and an energy beam isapplied for a short time, the relative temperature in the intermediaryportions of the plurality of members becomes low, and thereby crystalnuclei first start to form in the intermediary portions, which serves toprevent the disorderly formation of crystal nuclei.

In the present example, although Si or a compound of Si and Ge is givenas an example of the material for the p-Si film 403, other materials maybe employed. For example, other combinations of group IV elements suchas SiC, combinations of a group III element and a group V element suchas GaAs, combinations of a group II element and a group VI element suchas CdSe are also possible.

In addition, in the foregoing examples of the present invention, it isshown that Al is employed as the material for the gate electrode 405,the source electrode 407 s, and the drain electrode 407 d. However,other materials such as chromium (Cr), molybdenum (Mo), tantalum (Ta),titanium (Ti), and mixtures thereof may also be employed.

Further, the examples of the present invention employ an excimer layeras the energy beam for crystallizing the a-Si film 413, but other energybeams such as a laser light including Ar laser and YAG laser, ion beams,electron beams, and the like may be employed. By employing such energybeams as well, a high-density energy can be readily applied to targetedspots within a short time and therefore the crystallization can beperformed in a state where the substrate temperature is maintained at arelatively low temperature.

EXAMPLE 4-1

Example 4-1 employs a light beam having a distribution pattern such thata light energy intensity within a beam width (a light energy per unitarea, hereinafter simply referred to as a light intensity) monotonouslyincreases one side to the other or monotonously decreases from one sideto the other, in order to perform crystallization.

A typical distribution pattern such that a light intensity monotonouslyincreases one side to the other or monotonously decreases from one sideto the other is represented by the pattern having a linear lightintensity gradient as shown in FIG. 29(a), but the distribution patternmay be such that the light intensity increases or decreases in aconstant direction in an exponential function-like manner.

Examples of the light sources for the above-described light beam (notyet shaped) include various lasers such as a He-Ne laser, an argonlaser, a carbon dioxide gas laser, a ruby laser, and an excimer laser.The use of an excimer laser is preferred because a high output can beproduced and the beam is absorbed well by silicon. Now, a laserannealing method utilizing the excimer laser according to the presentinvention is described below.

FIG. 42 shows a schematic view illustrating the crystallizationprocedure using the laser annealing method. In FIG. 42, the referencenumeral 1400 designates a light beam-applying unit, and the numeral 1410designates an object to be irradiated with the light beam. The numeral1401 designates a laser light generator employing a XeCl excimer laser,for example, and the numeral 1402 a mirror, and the numeral 1403 a beamhomogenizer. The light beam-applying unit 1400 has such a constructionthat a light beam generated by the laser beam generator 1401 is guidedvia the mirror 1402 to the beam homogenizer 1403 in which the light beamis shaped into a predetermined light intensity pattern, and thenoutputted. The beam homogenizer 1403 has an optical system for shapinglight beam, and in the present example, a transmission filter (notshown) having a light transmissivity gradient as shown in FIG. 31 isprovided on the most downstream in the light path. Thus, the light beamgenerated in the laser beam generator 1401 transmits through thetransmission filter, whereby the light beam is shaped into a light beamhaving a pattern as shown in FIG. 29(a).

The above-described light beam-applying unit 1400 is capable ofoutputting a light beam having an average energy density (irradiationenergy per unit area) of 300 mJ/cm², an energy density in a low energydensity region L being 250 mj/cm², an energy density in a high energydensity region H being 350 mJ/cm², and a cross-sectional beam shapebeing 7 mm×7 mm. This light beam is applied to a surface to becrystallized, such as a surface of an amorphous silicon thin film, so asto crystallize the object to be crystallized.

More specific explanation for the process of the crystallization willfollow. First, as shown in the object 1410 to be irradiated in FIG. 42,a non-single crystalline silicon film 1412 having a film thickness of 85nm is deposited on the glass substrate 1411 by using, for example, areduced pressure CVD method. Specifically, the non-single crystallinesilicon film 1412 is formed by using monosilane gas (SiH₄) or disilanegas (Si₂H₆), setting the pressure at several torr, and heating the glasssubstrate 1411 at 350-530° C.

In the above-described process, an undercoat layer 1413 composed of, forexample, SiO₂ may be formed on the glass substrate 1411, and anon-single crystalline silicon film 1412 may be deposited over theformed undercoat layer 1413. The method for depositing the non-singlecrystalline silicon film 1412 is not limited to the reduced pressure CVDmethod, but for example, a plasma CVD method may be used. The filmthickness of the non-single crystalline silicon film 1412 is not limitedto 85 nm but may be appropriately determined.

In the non-single crystalline silicon film 1412 thus formed, a specificregion 1404 is irradiated and fused with 10 shots of, for example, theshaped excimer laser beam emitted from the light beam-applying unitlight 1400, and then allowed to dissipate heat to crystallize. In thepresent example, the irradiation with the light beam was carried out inthe following manner (see FIG. 43): when the light beam was irradiated,the object 1410 to be irradiated was placed in a sealed container havinga window made of quartz and the sealed container was evacuated (about10⁻⁶ torr), and the light beam was applied through the window to thespecific region 1404 at room temperature (about 23° C.). Note that thesealed container is not shown in FIG. 42.

The above-described conditions are exemplary and not restrictive exceptthat the light beam to be employed has a distribution pattern such thatthe light intensity within the beam width monotonously increases fromone side to the other or monotonously decreases from one side the other.For example, the light energy density may be other than described above,inasmuch as the light beam has the light intensity and light intensitygradient sufficient to crystallize the non-single crystalline siliconfilm 1412.

In addition, the degree of the light intensity gradient is notparticularly restrictive but may be selected in consideration of thequality, thickness and other properties of the non-single crystallinethin film so that the crystallization may be appropriately guided orcontrolled. Further, the beam width of the light beam to be applied andthe number of irradiations (the number of shots) are not limited to theabove-described conditions. For example, only one shot of a laser beamhaving a higher intensity may be applied.

In addition, the cross sectional shape of the light beam is notparticularly limited either, but may be, for example, a triangular shapeor a circular shape.

Referring now to FIGS. 29(a) to (g), the growth behavior of crystal inthe case of employing a light beam having a light intensity gradient isdescribed below.

When a light beam having a light intensity pattern as shown in FIG.29(a) is applied to the non-single crystalline silicon thin film, thetemperature on the irradiated surface shows a pattern such that thetemperature gradient slopes upwards in the central region and abruptlychanges in the peripheral regions, as indicated by the numeral 701(temperature distribution curve) in FIG. 29(b). The abrupt temperaturegradients are formed in the peripheral regions because the heatdissipated towards the outer periphery is large. Thereafter, when thelight irradiation is stopped, the temperature of the region in thevicinity of the crossing of the temperature distribution curve 701 andthe crystallization temperature line 702 (in the vicinity of theboundaries) first decreases below the melting point. Thereby,micro-crystals 704 are formed in the vicinity of the region (the numeral703 designates a cross sectional surface of the thin film).

Then, utilizing the crystals 704 as nuclei, crystal growth proceeds inthe rightward direction in the drawing where the temperature is stillabove the crystallization temperature. Unlike the case of the foregoingFIGS. 7(a) to 7(g), in this case of FIG. 29(b), a temperature gradientis formed in the central region, and therefore, heat flows from the hightemperature region (side H) into the low temperature region (side L),which heat serves to alleviate an abrupt temperature drop and to guidethe crystal growth towards the high temperature region (the right sidein the drawing). Accordingly, the formation and growth of crystal nucleiproceed smoothly, and as a result, uniformity in crystal grain sizes andcrystallinity is increased, and crystal grains longitudinally extendingfrom the L side towards the H side (in the orientation of crystalgrowth) are formed. In brief, when a light beam having a light intensitygradient is employed, it is made possible to produce a crystalline thinfilm having a high mobility in the orientation of crystal growth.

It is to be noted that when applying the light beam, the light applyingunit and the substrate may be in a fixed state (stationary state), oreither the light applying unit and the substrate may be moved orreciprocated. In the case of the light applying unit and the substratebeing moved or reciprocated, it is preferable that the movement be madein the direction of the light intensity gradient (in the direction offrom L to H or from H to L) as shown in FIG. 30. When moved orreciprocated in this direction, it is possible to finely control theorientation of crystal growth and increase the uniformity of the size ofcrystal grains and crystallinity. Furthermore, if the speed of moving isadjusted according to the degree of the light intensity gradient and thelight irradiation intensity, the orientation of crystal growth can befurther finely controlled.

It is noted that the arrows in FIG. 30 indicate the directions of themovement, the numerals 711 and 712 respectively indicate the regions tobe irradiated before and after the movement, and the numeral 713designates the repeatedly irradiated region (shaded area).

FIG. 30 illustrates an example in which the light beam is moved, but thesubstrate may be moved instead of the light beam. In addition, in thecase where a plurality of shots of light beam are applied, theirradiation may be performed with the irradiation position beingshifting or staggering by several percent to several tens percent of theirradiation area.

The poly-Si thin film produced in the manner described above isgenerally used for producing a TFT, with the central region being usedas a channel region, and a source region and a drain region formed byimplanting impurity ions such as phosphorus ions and boron ions intoboth side regions of the channel region. It is to be noted that thelight beam having the energy intensity pattern (FIG. 29(a)) described inthe present example is effective in the crystallization of a relativelynarrow region where a peripheral circuit for AM-LCD (active matrixliquid crystal display) and the like is to be formed.

EXAMPLE 4-2

Example 4-2 (as well as Example 4-3 to be described later) is effectivein the crystallization of a relatively wide area.

A light intensity distribution pattern of a light beam employed in thisexample is shown in FIG. 32(a). As shown in FIG. 32(a), the light beamaccording to Example 4-2 has a pattern such that a region H 721 having ahigh light intensity and a region L 722 having a low light intensity arealternately disposed on a plane. The ratio of the light intensities ofthe region H and the region L is not particularly limited, but may beappropriately determined. Generally, the total amount of the lightenergy is determined so that the entire surface to be irradiated (bothregions L and H) is fused within a prescribed number of shots of theirradiation. In the present example, the light intensity for the regionH was 300 mJ/cm², the light intensity for the region L was 200 mJ/cm²and the thickness of the amorphous silicon thin film was 50 nm. Otherconditions were the same as in the foregoing Example 4-1.

With reference to FIGS. 32(a) to 32(g), the crystallization behavior inthe present example is detailed below. First, when a light beam having alight distribution characteristic shown in FIG. 32(a) is applied, thetemperature on the surface of the thin film shows a distribution patternas shown in FIG. 32(b). In the process during which the temperature ofthe irradiated surface decreases after the light irradiation iscompleted, crystal nuclei 724 are formed in the position correspondingto the region L 722 at the time when the temperature of the region L 722approaches to the crystallization temperature line 723, as shown in FIG.32(c) (the numeral 725 shows a cross sectional surface of the thinfilm). As the temperature further decreases (FIG. 32(d)), the crystalgrowth is guided towards the high temperature regions H by the heatconducted from the high temperature regions H to the low temperatureregions L, and simultaneously, additional crystal nuclei are formed andgrown in the same manner (FIG. 32(e)).

Such formation and growth of crystals continue until the temperature ofthe high temperature region H decreases below the melting point (FIGS.32(f) and 32(g)), and in the present example, the orientation of crystalgrowth is guided in a direction from L towards H. Therefore, crystalgrains grow from each low temperature region L between the hightemperature regions H, and as a result, the crystal grains collide witheach other in the vicinity of the central region 726 in each hightemperature region H (FIG. 32 (g)). Thereby, a grain boundary is formedin the vicinity of the central region in each high temperature region H,and consequently the further crystal growth is guided in a directionparallel to the direction from L towards H. Accordingly, crystals aresomewhat grown in the direction perpendicular to the direction from Ltowards H, and as a result, crystal grains longitudinally extending inthe direction perpendicular to the direction from L towards H areformed.

Thus, according to the present example, it is made possible by theabove-described mechanism to smoothly perform crystallization of arelatively wide region to be irradiated, for example, of a severalcentimeter square region. Further, as has already been explained, whenthe light irradiation is performed by setting the light beam so that thedirection from L towards H results in a direction perpendicular to thedirection of the movement of carriers (a source-drain direction),carriers can move without crossing grain boundaries, hence achievinghigh speed TFTs.

In the present example as well as in the foregoing Example 4-1, thelight irradiation may be performed by moving (or reciprocating) eitherthe light beam or the substrate within the irradiation time (from t=t1to t=t2) as shown in FIG. 33. Thereby, the uniformity of crystallinitycan be further increased. It is noted that in FIG. 33, the numerals 731and 732 respectively designate the positions of the regions to beirradiated before and after the movement, the numeral 733 (shaded part)designates the repeatedly irradiated region, and the arrows indicate thedirections of the movement. The movement is, of course, not limited tosuch a movement.

It is also noted that such a light beam as shown in FIG. 32(a) in whicha portion having a strong light intensity H and a portion having a weakintensity L are arranged in a fringe pattern can be readily realized byknown techniques without requiring a special technique, and norestriction is placed on means for realizing such a light beam. One ofthe example of realizing such a light beam is as follows: pieces offilters that absorb the light to be applied to a certain degree arearranged at specific intervals so as to form a comb-like shapedtransmission filter having a transmission distribution as shown in FIG.34, and the filter is placed in a light path of the light beam-applyingunit (for example, in the beam homogenizer). Another example is a meanssuch that a filter composed of metallic fibers being arranged inparallel to each other in a longitudinal or latitudinal direction isdisposed in the light path. Further another example is a means in whichdiffraction interference is caused by providing slits in the light pathto produce a stripe-like shaped light intensity pattern.

EXAMPLE 4-3

Example 4-3 pertains to a method of imparting non-uniformity to thelight intensity distribution by utilizing light interference. Thismethod, as well as Example 4-2, can control the light intensitydistribution pattern relatively at will and therefore is suitable forcrystallization of a relatively wide area.

A light intensity distribution pattern of a light beam employed in thisexample is shown in FIG. 35. Such a light intensity distribution patterncan be readily formed by a means in which light interference is causedby simultaneously applying two coherent light beams 801 and 802 as shownin FIG. 36. Specifically, for example, a laser beam produced from alight ray is divided into two light paths by a semitransparent mirror,and a relative angle is formed by the light paths with the use of areflector, in order to cause interference.

In this connection, when two coherent lights are made to interfere witheach other, parts having a high light intensity (bright line parts H)and parts having a low light intensity (dark line parts L) are formed.The cycle of the interference pattern can be freely changed by adjustingthe angle formed at the crossing of the two light beams, and the degreeof modulation (that has an influence on the ratio of the light energyintensity of the bright line part to that of the dark line part) can bechanged easily by varying the energy intensities of the two light beams.Therefore, the interval and the intensity ratio of the bright line partsH and the dark line parts L can be set relatively freely. Thus, theinterval and intensity ratio are suitably set in consideration of thethickness and the like of the non-crystalline thin film, whose surfaceis to be irradiated.

With reference to FIGS. 37(a) to 37(g), the crystal growth behavior inthe case of employing the light beam having such an interference patternis now detailed below. It is noted that the conditions of the operationin the present example are the same as those in Example 4-1.

When the light beam characterized by FIG. 37(a) is applied to thenon-single crystal silicon thin film, a temperature distribution pattern(curve 901) in which the temperature is high in the bright line part Hand low in the dark line part L is formed on the thin film (FIG. 37(b)).In the process during which the temperature decreases after the lightirradiation is completed, crystal nuclei 903 are formed in the regionswhere the curve 901 first crosses the crystallization temperature line902 (where the temperature is the lowest in the low temperature regionL). As the temperature further decreases (FIG. 37(d)), the crystalgrowth is guided in a direction from L towards H by the heat conductedfrom the high temperature region H to the low temperature region L, andsimultaneously, additional crystal nuclei are formed grown in the samemanner (FIG. 37(e)). The formation and growth of crystals continue untilthe temperature in the high temperature region H, which corresponds tothe bright line part H, decreases below the melting point (FIGS. 37(f)and (g)).

Thus, according to Example 4-3, it is made possible by theabove-described mechanism to produce a crystalline semiconductor thinfilm having a uniform crystallinity and high field effect mobility in arelatively wide area. Furthermore, in the present example as well as inthe foregoing Example 4-2, a grain boundary (the line at which crystalgrains collide with each other) is formed in the central region of thehigh temperature region, and therefore, as described in the foregoingExample 4-2, a high field effect mobility can be obtained by employingas the direction of carrier movement a direction perpendicular to thedirection of the array of H→L→H→L.

EXAMPLE 4-4

Example 4-4 is basically similar to the foregoing Example 4-3. In thepresent example, however, the cycle of interference pattern and thedegree of modulation are dynamically adjusted so that the bright lineparts H and the dark line parts L change in a wave-motion-like manner.Now the detail of Example 4-4 is given below.

As shown in FIG. 38, at least one of the two coherent light beams issubjected to a dynamic phase modulation to form a light beam in whichthe positions of the bright line parts and the dark line parts in theinterference pattern change in a wave-motion-like manner. The phasemodulation is performed so that, for example, the phase of one of thelight beams is sequentially changed to be 0, π/2, π. . . relative to theother light beam. This chronologically shifts the positions of thebright line part and the dark line part in the interference pattern, andthus forms a light beam in which a striped pattern of the bright lineparts and the dark line parts changes in a wave-motion-like manner (FIG.38).

Examples of the means for the phase modulation include a method suchthat, by using a mirror, the light path length of one of the two lightbeams is dynamically changed to cause the phase modulation, and a methodsuch that the reflectivity of a transparent material disposed in thelight path is dynamically changed. Such optical systems are incorporatedin the foregoing beam homogenizer (the reference numeral 1403 in FIG.42), for example.

The present example is very effective in guiding the crystal growth in aspecific direction since the temperature distribution pattern on thesurface of the thin film to be irradiated becomes such that the hightemperature region H and the low temperature region L replace each otherin a wave-motion-like manner. Moreover, the method has an additionaladvantageous effect that impurities are expelled from the effectiveregion, and thereby can increase the purity of the thin film, achievinga high quality crystalline thin film. Such an effect of expellingimpurities from the effective area is based on the following principle.The thin film component and the impurities have different physicalproperties such as melting points and specific gravities, and thewave-motion-like temperature variation causes a difference in proceedingvelocity between the two. Therefore, by repeating the irradiation anumber of times, the impurities present in very small amounts areseparated from the thin film component.

It is noted here that the cycle of the interference pattern and thedegree of modulation may be adjusted during one shot of the irradiationor each time of a number of shots of the irradiation. In addition, thedegree of modulation may be controlled depending upon each of the stagesin the crystal growth. By employing these, the crystal growth can bemore suitably guided.

In both of Example 4-3 and 4-4, the irradiation may be performed whileeither the laser light or the substrate is being moved (or reciprocated)to more appropriately control the crystal growth, as in the foregoingExample 4-1 or 4-2. When the movement is directed in a the directionparallel to the direction of the fringe pattern made of the bright lineparts H and the dark line parts L, the effect of expelling impuritiescan be obtained by the foregoing Example 4-3 as well.

In the above-described example, the temperature distribution is made inthe surface direction of the irradiated region, but it is also possibleto form the light intensity distribution in the thickness direction ofthe thin film to be irradiated, as shown in FIG. 39. FIG. 39schematically shows an object to be irradiated being irradiated with alight beam, the object in which a thin film of an amorphous siliconlayer 1101, an undercoat layer 1102 (SiO₂), and a glass substrate 1103are layered in order of the direction of the irradiation (from above inthe drawing). When a light beam having a light intensity distributionalong the vertical direction (thickness direction) enters the object tobe irradiated, a temperature distribution corresponding to the waveformis formed in the thickness direction. However, silicon thin films usedfor TFTs generally have a thickness as small as several tens ofnanometers, which is smaller than the periodic cycle of the interferencepattern, and therefore, it is difficult to form a periodic temperaturedistribution in the thickness direction.

Yet, the upper surface of the thin film 1101 is cooled by heat radiationto the surrounding environment, and the lower surface (the substrateside) is also cooled by heat conduction to the undercoat layer 1102 andthe glass substrate 1103. This indicates that a temperature distributionis also present in the thickness direction, and it is possible toenhance this temperature distribution. In order to enhance thetemperature distribution in the thickness direction, the above-describedinterference. pattern can be utilized. Specifically, for example, theinterference can be caused by a reflector provided on the lower surfaceof the glass substrate 1103. Or alternatively, the reflectivitydifference between the thin film 1101 and the undercoat layer 1102 orbetween the thin film 1101 and the glass substrate 1103 is increased tocause the interference between the light incoming from the direction ofthe thin film and the light reflected at the interface of each layer. Inaddition, by adjusting the periodic cycle of the interference pattern,the temperature distribution can be formed in the thickness direction,and thereby the crystal growth in the thickness direction can becontrolled.

In the case of controlling the temperature distribution in the thicknessdirection, it is desirable to specifically determine suitable conditionsfor each case taking into consideration the thickness of the non-singlecrystalline thin film and the thermal conductivities of the undercoatlayer and the substrate. As another variation, it is possible that alight emitted from a single light source is divided into two lights, andone is applied from the direction of the surface thin film (from above)whilst the other is applied from the direction of the substrate (frombelow), whereby the interference is caused inside the thin film. Notethat in this case, the substrate and the undercoat layer must becomposed of a material capable of transmitting the light beam.

EXAMPLE 4-5

The present example is characterized in that, by suitably setting thepressure of the ambient gas in the process of crystallization, atemperature gradient is formed on a surface to be crystallized. In thepreceding Examples 4-1 to 4-4, by contrast, the light intensity patternof the light beam is adjusted or controlled, and thereby crystallinityand uniformity of crystals are improved. Accordingly, the approach ofthe present example is completely different from those in the Examples4-1 to 4-4. The present example will now be detailed below.

FIG. 40 shows a cross sectional view of the object to be irradiated(layered material) as shown in FIG. 39. In FIG. 40, the referencenumeral 1200 designates the surface to be irradiated with the lightbeam, the numeral 1201 a thin film, the numeral 1202 an undercoat layer,and the numeral 1203 a substrate. The arrows indicate the directions ofheat conduction (directions of heat radiation). As shown in FIG. 40,although part of the heat diffuses in the ambient atmosphere (in theupward direction) and in the outward directions from the irradiatedregion in the thin film (in the rightward and leftward directions in thefigure), most of the heat is conducted towards the substrate (in thedownward direction), where the contact area is large and the thermalconductivity is high. In this connection, prior art laser annealingmethods are performed in highly evacuated atmosphere, and employs alight beam having a uniform distribution of light intensity. Therefore,as described in the foregoing Example 4-1, a temperature gradient isdifficult to be formed in the central region of the irradiated surface,and hence crystal nuclei are difficult to be formed at the early stageof cooling. In addition, a large number of crystal nuclei start to beformed simultaneously at a certain stage of the process of cooling,which is undesirable.

Unlike such prior art methods, the present example is characterized inthe ambient atmosphere is not evacuated to a high degree, and that byutilizing motion of the molecules of the gas constituting the ambientatmosphere, a region not uniform in temperature is produced in thesurface irradiated with a light.

First, the principle of this Example 4-5 will be discussed. The time ofone shot (one pulse) of such a pulsed light as an excimer laser isgenerally as short as 20 to 50 nanoseconds. Therefore, it is necessarythat within such a short irradiation time, silicon or the like be heatedabove the melting point, and this requires that silicon or the likeshould generally be an extremely thin film with a thickness as thin asseveral tens of nanometers. Such extremely thin films tend to be greatlyinfluenced by the ambient gas molecules in the process of cooling.

The gas molecules present in the thin film and the gas moleculesconstituting the ambient atmosphere are performing such a motion thatthe molecules collide with and depart from the surface of the thin filmwith a certain probability. The level of the thermal energy of the gasmolecules is lower than that of the irradiated and heated thin film, andtherefore when the molecules collide with and depart from the surface ofthe thin film, they take away the heat of the thin film.

From such an effect of the gas molecules, it is considered that aperturbation-like temperature distribution must be formed on the thinfilm. If, therefore, the pressure of the ambient atmosphere and the typeof gas molecules constituting the ambient atmosphere are selectedappropriately, a region not uniform in temperature (a low temperatureregion) can be formed, in the irradiated region, even in the case wherethe region is irradiated with a light beam having a uniform lightintensity distribution. It is considered that if such a region can beformed, the formation of crystal nuclei and smooth crystal growth can berealized. On the basis of this consideration, the following experimentswere performed.

It is to be noted that in this connection, the crystal nuclei in theprocess of crystallization play a role analogous to that by water vapornuclei when moisture in the atmosphere forms dropwise condensation.

EXPERIMENT 1

Preparation

First, an SiO₂layer (undercoat layer) having a thickness of 200 nm wasformed on a substrate (thickness being 1.1 mm) of Corning #7059 glass.Then, an amorphous silicon thin layer having a thickness of 50 nm wasformed over the undercoat layer. Thus an object to be irradiated wasprepared. The object to be irradiated provided with the amorphoussilicon thin layer 1503 was then placed in a sealed container 1500having a window 1501 made of quartz glass, as shown in FIG. 43. The airin the sealed container 1500 was evacuated and then hydrogen gas wasintroduced from a hydrogen gas cylinder 1502 to bring the pressure ofhydrogen gas in the sealed container up to predetermined levels. Then,an excimer laser generated by a laser irradiation unit 1510 was appliedto the amorphous silicon layer 1503 of the object to be irradiatedthrough the window 1501. Thereafter, the object was cooled forcrystallization.

The following pressures were employed as the predetermined pressure(ambient pressure) of the hydrogen gas: 5×10⁻⁶ torr, 1×10⁻⁵torr, 1×10⁻²torr, 1×10⁻¹ torr, 1 torr, and 10 torr. The conditions of the laserirradiation were as follows: the employed light beam was a conventionallight beam having a uniform light intensity distribution, 1 pulse (1shot) of the light beam was 30 n.sec., the beam width was 7 mm×7 mm, andthe light intensity was 350 mJ/cm². This light beam was applied 100pulses. Then, the object to be irradiated was allowed to cool in theenvironment at room temperature to polycrystallize the amorphous siliconlayer 1503.

It is noted that in FIG. 43, the numeral 1511 designates an excimerlaser generator, the numeral 1512 a mirror and the numeral 1513 a beamhomogenizer.

Relationship between Ambient Pressure and Crystallinity

Six samples of the crystalline silicon thin film (poly-Si) preparedunder the above-described conditions were evaluated by visualobservation using an oblique ray. In addition, the Raman intensity wasmeasured by micro-Raman spectroscopy, and crystallinities of those sixsamples were evaluated assuming the Raman intensity in the case of ahydrogen gas pressure being 5×10⁻⁶ torr as 1. The results are shown inTable 1 below.

TABLE 1 Ambient pressure* 10⁻⁶ 10⁻⁵ 10⁻² 10⁻¹ 1 10 Visual SlightNoticeable Very strong Considerable Considerable Considerableobservation scattering scattering scattering scattering scatteringscattering result Bluish Greenish Pure white Whitish Whitish WhitishRaman 1 4 7 6 6 6 intensity (relative value) *torr

As shown in Table 1, visual observation confirmed that the state of thesilicon thin film after the laser annealing changes corresponding to theambient pressure in manufacturing. Specifically, when the hydrogen gaspressure (ambient pressure) was in the order of 10⁻⁶ torr, a slightbluish scattered light was observed, whereas when the pressure wasincreased to the order of 10⁻⁵ torr, the scattered light was shiftedtowards the green side and the entire region was rendered brighter. Whenthe hydrogen gas pressure was further increased to the order of 10⁻¹,the scattering became strong, with a whitish opaque state, and almostthe same state was observed up to the order to about 10 torr.

In the evaluation of crystallinity by micro-Raman spectroscopy, thesample crystallized under a hydrogen gas pressure of 1×10⁻⁵ torrexhibited four times as high the Raman intensity as that of the sampleprepared under a pressure of 5×10⁻⁶ torr. The samples crystallized undera pressure of 1×10⁻⁵ torr to 10 torr exhibited 6 to 7 times as high.These results confirm the following.

Conventionally, it has been a usual practice that the light irradiationis performed with the ambient pressure reduced as low as possible (to ahighly evacuated state), except for special cases such as where themolecules in the ambient atmosphere are to be reacted with the thinfilm. However, as evident from Table 1, a high degree of crystallinitycannot be obtained under such a highly evacuated state. By contrast, thecrystallinity improves as the hydrogen gas pressure increases. Theresults of the experiment show that for crystallization by a laserannealing treatment, it is desirable that the ambient atmospherepressure be higher than a certain value, preferably 1×10⁻² torr orhigher.

It is considered that the reason why a high degree of crystallinity cannot be obtained under a highly evacuated state is that aperturbation-like uneveness in temperature cannot be formed by themotion of the gas molecules. By contrast, it is considered that theimprovement of crystallinity in the case where the pressure of hydrogengas exists is because when the hydrogen molecules collide with anddepart form the surface of the thin film, the molecules remove the heatfrom the thin film, forming a local and perturbation-like uneveness intemperature. In short, the results shown in Table 1 prove the abovediscussion.

EXPERIMENT 2

Samples of crystalline silicon thin film (poly-Si) were prepared in thesame manner as in Experiment 1 except that hydrogen gas pressuresemployed as ambient atmosphere pressures were 5×10⁻⁶ torr and 1 torr,and that the light beam was applied 1, 10, 100, and 500 times under therespective hydrogen gas pressures. Then, in the same manner as describedin Experiment 2, the Raman intensities were measured, and the effects ofhydrogen gas pressures were evaluated on the basis of the relationshipbetween the number of irradiations and the crystallinity. The resultsare shown in FIG. 41.

As apparent from FIG. 41, it was found that in the case of a hydrogengas pressure of 1 torr, the Raman intensity increased and thecrystallinity improved as the number of irradiations increased. Bycontrast, in the case of a hydrogen gas pressure of 5×10⁻⁶ torr, theRaman intensity did not increase and the crystallinity did not improveeven when the number of time of irradiation increased over 10.

The results indicate that the ambient atmosphere should not be highlyevacuated also in a method in which the light beam is applied amultiplicity of times to cause crystallization. It was also confirmed bythe results that, by at least setting the hydrogen gas pressure to be 1torr, the crystallinity can be improved as the number of times ofirradiation is increased.

While the test results under the condition of the pressure exceeding 10torr are not shown in the foregoing Experiment 1, it is considered thata high-quality crystal thin film can be formed also under the conditionof the pressure exceeding 10 torr. The reason is as follows. It isconsidered that as the hydrogen gas pressure increases, the number ofhydrogen molecules colliding with and departing from the surface of thethin film increases, which degrades the effect of causing anon-uniformity in temperature distribution. Nevertheless, another effectis obtained, such as in the following. When the thin film is heated to atemperature over the melting point by irradiation, the vapor pressureinside the thin film increases, and the increased vapor pressure causessuch disadvantages that crystal growth is hindered and substancesconstituting the thin film are dispersed. However, if the ambientpressure is high, the high ambient pressure suppresses the dispersion ofthe substances constituting the thin film, and as a result, the crystalgrowth can proceed smoothly.

It is noted here that although hydrogen gas (H₂), which has a highspecific heat and is effective in cooling heat, was employed as a gasconstituting the ambient atmosphere in Experiments 1 and 2, but the gasconstituting the ambient atmosphere is not limited to hydrogen gas.Examples of the gas include such inert gases as N₂, He, and Ar, and amixed gas in which the molecules of two or more of these gases aremixed. However, molecules of different types of gases have differenteffects (including adverse effects) on the specific heat and the thinfilm substances, and therefore, it is preferable to set a suitable gaspressure according to the types of gas molecules.

It is also noted that while the excimer laser is used as a light beam inthe above examples, the light beam usable in the present invention isnot limited to the excimer laser. Other suitable examples include notonly continuous-wave lasers such as a He—Ne laser and an argon laser butalso a light of an ultraviolet lamp.

It is also to be noted that the present invention is particularlyeffective as a method of polycrystallization, but needless to say, isalso applicable to a method of crystallization of single crystals.

Further, while the present invention has been described with particularemphasis on the process of forming crystalline semiconductor thin film,it is to be understood that the present invention is widely applicableto modification of substances using light energy, such as melt moldingand beat annealing of alloys.

EXAMPLE 5-1

Example 5-1 of the present invention will be detailed with reference toFIGS. 44(a), 44(b), and FIG. 45.

First, as shown in FIGS. 44(a) and 44(b), an amorphous silicon thin film522 as a precursor semiconductor thin film is formed on a glasssubstrate 521 by a plasma CVD method. The thickness of the amorphoussilicon thin film 522 is not particularly restricted, and is generallyvaried depending on the application. For example, the thin films for usein TFTs are formed to have a thickness of about 300 to 1000 Å, whereasthe thin films for photosensors or photovoltaic devices such as solarcells are to have a thickness of about 1 μm or greater.

Second, the glass substrate 521 having the amorphous silicon thin film522 formed thereon is placed on a substrate stage 535, and the amorphoussilicon thin film 522 is irradiated for 5 seconds in a stationary statewith a first energy beam for crystallization, a laser beam 531 a of anXeCl excimer laser 531, and a second energy beam for preheating, a laserbeam 532 a of an Ar laser 532.

More particularly, the XeCl excimer laser 531 has an oscillationfrequency of 50 Hz, a wavelength of 308 nm, and an irradiation energy of300 mJ/cm². The Ar laser 532 is a continuous wave laser having awavelength of 488 nm and an output power of 20 W/cm². The laser beam 531a transmits through a semitransparent mirror 533, whereas the laser beam532 a is reflected by the semitransparent mirror 533. In the amorphoussilicon thin film 522, the regions 531 b, 532 b to be irradiatedrespectively with the laser beams 531 a, 532 a have a belt-like shape,and the laser beam 532 a is applied to the region 532 b, which is widerthan and includes the region 531 b to be irradiated with the laser beam531 a.

As described above, it is preferable that the laser beams 531 a, 532 abe applied perpendicular to the amorphous silicon thin film 522, such asby using the semitransparent mirror 533, from such a viewpoint that thenon-uniformities in crystal grain sizes and field effect mobilities areeasily reduced. However, the laser beams are not necessarily applied atexactly right angles, but may be applied substantially perpendicular tothe amorphous silicon thin film 522, such as by slightly staggering twomirrors. In addition, as an alternative to the XeCl excimer laser 531,various other lasers having a wavelength of 400 nm or less, such as anArF, KrF, or XeF excimer laser may be employed, and as an alternative tothe Ar laser 532, various other lasers having a wavelength of 450-550 nmmay be employed.

The amorphous silicon thin film 522 has, in the case of the filmthickness being 1000 Å for example, a transmissivity characteristic asshown in FIG. 1. Specifically, the absorption coefficient for a lighthaving a wavelength of about 500 nm is about 10⁵ cm⁻¹, which is aboutthe reciprocal number of the film thickness, and the absorptioncoefficient for a light having a wavelength of less than 400 nm is 10⁶cm⁻¹, which indicates that little of the light transmits through thefilm. Accordingly, on one hand, most of the laser beam 531 a of the XeClexcimer laser 531, which has a wavelength of 308 nm, is absorbed nearthe surface of the amorphous silicon thin film 522, and by the resultingtemperature increase and the conduction of the heat, mainly theamorphous silicon thin film 522 is heated to about 1200° C. On the otherhand, the laser beam 532 a of the Ar laser 532 having a wavelength of488 nm is absorbed in almost the entire region throughout the thicknessdirection of the amorphous silicon thin film 522, and by the conductedheat, the glass substrate 521 is heated to about 400° C. Thereby, afterbeing irradiated with the laser beams 531 a and 532 a, the amorphoussilicon thin film 522 is annealed and crystal growth is promoted. Thus,a polysilicon thin film 523 having large crystal grains is formed.

The polysilicon thin film 523 thus formed and a prior art polysiliconthin film crystallized by using the XeCl excimer laser 531 alone weresubjected to Raman spectroscopy to evaluate the conditions ofcrystallization. The measurement result for each of the thin films isshown in FIG. 45, respectively indicated by the reference character P orR. As is evident from FIG. 45, it was confirmed that a higher Ramanscattering intensity and a better crystallinity was obtained in the case(P) where both the laser beam 531 a of XeCl excimer laser 531 and thelaser beam 532 a of the Ar laser 532 were employed than in the case (R)where the laser beam 532 a of the Ar laser 532 alone was employed.

In another test, the laser beam 531 a, 532 a were also applied to theentire area of the amorphous silicon thin film 522 while moving theglass substrate 521 provided with the amorphous silicon thin film 522 ata speed of, for example, 3 mm/sec., and a plurality of regions of theformed polysilicon thin film 523 were subjected to Raman spectroscopy toevaluate a distribution of crystallization. The test result confirmedthat a remarkably high degree of uniformity was achieved.

It is noted here that in order to enhance the uniformity ofcrystallization, it is preferable that the region 532 b to be irradiatedwith the laser beam 532 a be a larger region than the region 531 b to beirradiated with the laser beam 531 a, and include the region 531 b.

EXAMPLE 5-2

As shown in FIG. 46, an infrared lamp 534 having a wavelength of forexample 4 μm may be employed in place of the Ar laser 532 employed inthe foregoing Example 5-1. The glass substrate 521 has a transmissivitycharacteristic as shown in FIG. 47, and therefore infrared ray 534 aemitted from the infrared lamp 534 transmits through the amorphoussilicon thin film 522, most of the ray being absorbed by the glasssubstrate 521. When the laser beam 531 a of the XeCl excimer laser 531is applied thereto, mainly the amorphous silicon thin film 522 is heatedby the laser beam 531 a, whereas mainly the glass substrate 521is-heated by the infrared ray 534 a of the infrared lamp 534, as in ananalogous manner to that in the foregoing Example 5-1. Thereby, afterbeing irradiated with the laser beam 531 a and the infrared ray 534 a,the amorphous silicon thin film 522 is annealed and crystal growth ispromoted. Thus, a polysilicon thin film 523 having large crystal grainsis formed.

The polysilicon thin film 523 formed under the same conditions as in theforegoing Example 5-1except that the infrared lamp 534 was employed wassubjected to Raman spectroscopy. The measurement result is shown in FIG.45, designated by the reference character Q. As is evident from the FIG.45, it was confirmed that the Raman scattering intensity andcrystallinity were higher than those in the case (R) where the laserbeam 531 a of the XeCl excimer laser 531 alone was employed.

It was also proved that uniformity of the crystal grains is excellent,as in the foregoing Example 5-1.

In addition to the infrared ray 534 a, the laser beam 532 a of the Arlaser 532 may also be employed, as in the foregoing Example 5-1.Further, the infrared ray 534 a as well may be applied perpendicular tothe amorphous silicon thin film 522, such as by using a semitransparentmirror as in the foregoing Example 5-1.

EXAMPLE 5-3

Example 5-3 of the present invention is now detailed with reference toFIGS. 48 through 50.

First, as shown in FIG. 48, a microcrystalline silicon thin film 524 asa precursor semiconductor thin film is formed on a glass substrate 521by using an inductively-coupled plasma CVD method. Specifically, forexample, a microcrystalline silicon thin film 524 having a thickness of85 nm is formed by using as the reaction gas a 2:3 mixture of monosilanegas (SiH₄) and hydrogen gas under the reaction conditions of thesubstrate temperature (reaction temperature) being 350-530° C. and thepressure being several torr. It is noted that in place of themicrocrystalline silicon thin film 524, the amorphous silicon thin film522 may be formed, as in the foregoing Example 5-1. Other systems usableas an alternative to the plasma CVD system include an LP (low power) CVDsystem and a sputtering system.

Second, the glass substrate 521 provided with the microcrystallinesilicon thin film 524 was heat-treated for 30 minutes or longer at400-500° C., as a dehydrogenation treatment in which hydrogen in themicrocrystalline silicon thin film 524 is removed. The treatment isintended to prevent a damage to the microcrystalline silicon thin film524 resulting from the rapid release of the hydrogen incorporated intothe microcrystalline silicon thin film 524 during the laser annealingdescribed below.

Third, a laser annealing is performed. Specifically, as shown in FIG.49, the glass substrate 521 is placed in a chamber 541 provided with anirradiation window 541 a made of a quartz plate. The glass substrate 521is then irradiated with the laser beam 531 a of the XeCl excimer laser531 and an incandescent light 542 a of an incandescent lamp 542 tocrystallize the microcrystalline silicon thin film 524. Thus, apolysilicon thin film 523 is formed. More specifically, the laser beam531 a is 10 shots of a pulsed laser having a pulse width of several tensof nanoseconds, a wavelength of 308 nm, and an irradiation energy 350mJ/cm². The laser beam 531 a is applied via a laser beam attenuator 543,a homogenizer (apparatus for homogenizing the laser beam) 544, and areflector 545. The incandescent light 542 a is applied so that themicrocrystalline silicon thin film 524 is heated to about 400° C.

Thereafter, the polysilicon thin film 523 is heated to 350° C. or higherin a hydrogen plasma atmosphere to perform a hydrogenation treatment bywhich dissociated dangling bonds in the polysilicon thin film 523 areterminated by hydrogen.

The crystal grain size in the polysilicon thin film 523 thus formed wasmeasured by using SEM and TEM. The grain size was found to be 0.7 μm,and it was thereby confirmed that the grain size increased over thegrain size in the prior art polysilicon films, which was 0.3 μm. Thefield effect mobility also increased to 80 cm²/V·sec, which was also animprovement over 50 cm²/V·sec of the prior art. Furthermore, the totalof the density of defects in the interface and the inside of thepolysilicon thin film 523 was reduced to 1.0×10 ₁₂ cm⁻²eV⁻¹, from1.3×10¹² cm⁻²eV⁻¹ of the prior art. This demonstrates that the use ofthe incandescent lamp 542 in combination with the irradiation with thelaser beam 531 a increases the crystal grain size and improves the filmquality in the polysilicon thin film 523.

The conditions in applying the laser beam 531 a were variedexperimentally. As a result, it was found that crystallization takesplace when the irradiation energy is 200 mJ/cm² or greater, whereasmicrocrystalline silicon disappears when the irradiation energy is 500mj/cm² or greater. In addition, it was found that in the range of 300mj/cm² to 450 mJ/cm², sufficient crystal growth takes place and a largegrain size results. It was also found that when the number of times ofthe irradiation is five times or more, the occurrence of crystal defectsis suppressed and crystallinity is improved.

Thereafter, a thin film transistor (TFT) is prepared by, for example,forming predetermined insulating films and conductive films, performinga patterning by etching, and then performing an ion implantation. Thepatterning of the polysilicon thin film 523 may be performed before thelaser annealing.

The gate voltage (Vg)—drain current (Id) characteristic of the TFT thusprepared was measured. As shown in FIG. 50, the TFT of the presentexample showed that the rise of the drain current versus gate voltagebecame more abrupt, which confirms that the subthreshold characteristicwas improved. The threshold voltage was also reduced.

It is noted that, in place of simultaneously applying the laser beam 531a and the incandescent light 542 a as described above, the irradiationmay be performed in the following manner as shown in FIG. 51. That is,the glass substrate 521 is placed on a substrate stage 535 capable ofmoving in the horizontal direction, and the incandescent light 542 a isapplied to a more forward position in the microcrystalline silicon thinfilm 524 with respect to a direction of moving of the substrate than aposition where the laser beam 531 a is applied, while the substratestage 535 is being moved in the direction indicated by the arrow A inFIG. 51. Thereby, heating by the incandescent light 542 a is performedbefore the heating by the laser beam 531 a. This also achieves the sameadvantageous effects.

EXAMPLE 5-4

A TFT of Example 5-4 according to the present invention will now bedescribed with reference to FIGS. 52 and 50.

First, an amorphous silicon thin film 522 is formed on the glasssubstrate 521 by a plasma CVD method, as shown in FIG. 52. Morespecifically, for example, an amorphous silicon thin film 522 having athickness of 85 nm is formed by using as reaction gas a mixture ofmonosilane gas (SiH₄) and hydrogen gas under the reaction conditions ofthe substrate temperature being 180-300° C. and the pressure being 0.8torr.

Second, the glass substrate 521 provided with the amorphous silicon thinfilm 522 is subjected to a dehydrogenation treatment in the same mannersas in the foregoing Example 5-3.

Third, the amorphous silicon thin film 522 is irradiated with the laserbeam 531 a of the XeCl excimer laser 531 and an excimer lamp light 551 aof an excimer lamp 551, while the glass substrate 521 is being moved inthe direction of the arrow A in FIG. 52. Thus, the amorphous siliconthin film 522 is crystallized and a polysilicon thin film 523 is formed.Specifically, the laser beam 531 a has an irradiation energy of 350mj/cm², and is applied in such a manner that the irradiated region 531 bin the amorphous silicon thin film 522 result is a 500 μm×70 mmbelt-like shape. In addition, accompanying with the movement of theglass substrate 521, each shot of the laser beam 531 a is applied sothat the region 531 b to be irradiated overlaps with 90 percent of thepreceding already-irradiated region, and the entire region of theamorphous silicon thin film 522 is irradiated 10 times with the laserbeam 531 a. Meanwhile, the excimer lamp light 551 a is a light in therange from visible rays to ultraviolet rays, and is applied directly andvia a concave reflector 552 on a 5 mm×70 mm region 551 b to beirradiated, which includes the region 531 b to be irradiated with thelaser beam 531 a, so as to heat the amorphous silicon thin film 522 toabout 500° C.

Thereafter, a hydrogenation treatment is performed as in the foregoingExample 5-3.

The crystal grain size in the polysilicon thin film 523 thus formed wasmeasured by using SEM and TEM. The grain size was found to be 1 μm, andit was thereby confirmed that the grain size increased over the grainsize in the prior art polysilicon films, which was 0.3 μm. The fieldeffect mobility also increased to 120 cm²/V·sec, which was also animprovement over 50 cm²/V·sec of the prior art. Furthermore, the totalof the density of defects in the interface and the inside of thepolysilicon thin film 523 was reduced to 1.1×10¹² cm⁻²eV⁻¹, from1.3×10¹² cm⁻²eV⁻¹ of the prior art. This demonstrates that the use ofthe excimer lamp 551 in combination with the irradiation with the laserbeam 531 a increases the crystal grain size and improves the filmquality in the polysilicon thin film 523.

Thereafter, as in the foregoing Example 5-3, a TFT is prepared by, forexample, forming predetermined insulating films and conductive films,performing a patterning by etching, and then performing an ionimplantation.

The gate voltage (Vg)-drain current (Id) characteristic of the TFT thusprepared was measured. As shown in FIG. 50, the TFT of the presentexample as well showed that the rise of the drain current versus gatevoltage became more abrupt, which confirms that the subthresholdcharacteristic was improved. The threshold voltage was also reduced to4.2 V, from 5.0 V of the prior art.

EXAMPLE 5-5

Example 5-5 of the present invention will now be detailed with referenceto FIGS. 53 and 50.

First, an amorphous silicon thin film 522 is formed on the glasssubstrate 521, followed by the dehydrogenation treatment performed inthe same manner as in the foregoing Example 5-4.

Subsequently, as shown in FIG. 53, while the glass substrate 521 isbeing moved in the direction of the arrow A, the laser beam 531 a of theXeCl excimer laser 531 and the excimer lamp light 551 a of the excimerlamp 551 are applied, and in addition, with the use of a heater 561, theglass substrate 521 was heated from the bottom side, thus forming apolysilicon thin film 523. Specifically, the present example isidentical to the foregoing Example 5-4 in the conditions of irradiationwith the laser beam 531 a and the excimer lamp light 551 a, but differsin that the entire glass substrate 521 is heated to 450° C. with the useof the heater 561 in addition to the irradiation.

Thereafter, a hydrogenation treatment is performed as in the foregoingExample 5-3.

The crystal grain size in the polysilicon thin film 523 thus formed wasmeasured by using SEM and TEM. The grain size was found to be 1.5 μm,and it was thereby confirmed that the grain size increased over thegrain size in the prior art polysilicon films, which was 0.3 μm. Thefield effect mobility also increased to 150 cm²/V·sec, which was also animprovement over 50 cm²/V·sec of the prior art. Furthermore, the totalof the density of defects in the interface and the inside of thepolysilicon thin film 523 was reduced to 8.7×10¹¹ cm⁻²eV⁻¹, from1.3×10¹² cm⁻²eV⁻¹ of the prior art. This demonstrates that the use ofthe excimer lamp 551 and the heater 561 in combination with theirradiation with the laser beam 531 a further increases the crystalgrain size and improves the film quality in the polysilicon thin film523.

In addition, a TFT was produced as in the foregoing Example 5-3 and thegate voltage (Vg)-drain current (Id) characteristic of the TFT thusprepared was measured. As shown in FIG. 50, the TFT of the presentexample showed that the rise of the drain current versus gate voltagebecame further more abrupt, which confirms that the subthresholdcharacteristic was improved.

The temperature of the glass substrate 521 was varied experimentally. Asa result, it was found that the improvement in crystal quality wasobtained when the temperature of the glass substrate 521 was 300° C. orhigher, whereas deformation occurs in the glass substrate 521 andthereby the fabrication of TFTs and the like becomes difficult when thetemperature was 600° C. or higher.

EXAMPLE 5-6

Example 5-6 of the present invention will now be detailed with referenceto FIGS. 54 and 50.

First, an amorphous silicon thin film 522 is formed on the glasssubstrate 521, followed by the dehydrogenation treatment performed inthe same manner as in the foregoing Example 5-4.

Subsequently, as shown in FIG. 54, while the glass substrate 521 isbeing moved in the direction of the arrow A, the laser beam 571 a of aKrF excimer laser 571 and the excimer lamp light 551 a of the excimerlamp 551 are applied, and in addition, with the use of the heater 561,the glass substrate 521 was heated from the bottom side, thus forming apolysilicon thin film 523. Compared with the foregoing Example 5-5, thepresent example differs from the foregoing Example 5-5 mainly in thatthe excimer lamp light 551 a is applied from directly above the glasssubstrate 521, transmitting through a wavelength selective reflector572, and that the KrF excimer laser 571 is used in place of the XeClexcimer laser 531 and the laser beam 571 a is applied via the wavelengthselective reflector 572. In addition, the excimer lamp light 551 a isapplied to a 5 mm×100 mm region 551 b to be irradiated including aregion 571 b to be irradiated with the laser beam 571 a. Otherconditions such as those of heating are identical to the foregoingExample 5-5.

The wavelength selective reflector 572 used here reflects a light havinga wavelength shorter than 280 nm, but transmits a light having awavelength longer than 280 nm. Therefore, the laser beam 571 a (thewavelength being 248 nm) of the KrF excimer laser 571, in which KrF isused for discharge, is reflected by the wavelength selective reflector572, and then applied substantially perpendicular to the amorphoussilicon thin film 522. At the same time, the excimer lamp light 551 a inthe range from visible rays to ultraviolet rays transmits through thewavelength selective reflector 572, and is applied substantiallyperpendicular to the amorphous silicon thin film 522.

Thereafter, a hydrogenation treatment is performed as in the foregoingExample 5-3.

The polysilicon thin film 523 thus formed by applying the laser beam 571a and the excimer lamp light 551 a perpendicular to the amorphoussilicon thin film 522 exhibited a crystal grain size of 1.5 μm, a fieldeffect mobility of 150 cm²/V sec, and a density of defects of 8.7×10¹¹cm²/eV⁻¹, all of which were the same as those in the foregoing Example5-5. The uniformities of the crystal grain size and field effectmobility in the respective regions of the polysilicon thin film 523 werefurther increased, and uniform characteristics were obtained throughoutthe entire surface of the polysilicon thin film 523.

In addition, a TFT was produced as in the foregoing Example 5-3 and thegate voltage (Vg)-drain current (Id) characteristic of the TFT thusprepared was measured. As shown in FIG. 50, the TFT of the presentexample showed the same characteristics as in the foregoing Example 5-5.

The selective reflection or transmission of laser beams and the like bythe wavelength selective reflector 572 depending upon the wavelengthscan be readily realized by using the KrF excimer laser 571, as describedabove. However, the present example is not limited thereto, but othershort wavelength lasers employing XeBr, KrCl, ArF, ArCl, or the like maybe employed.

EXAMPLE 5-7

Example 5-7 of the present invention will now be detailed with referenceto FIGS. 54 to 56 and 50.

The present example 5-7 differs from the foregoing Example 5-6 in thatthe region 551 b to be irradiated with the excimer lamp light 551 a is 5mm×70 mm, and the heating temperature of the amorphous silicon thin film522 by the excimer lamp 551 is set at various temperatures. Otherheating conditions and the like are identical to the foregoing Example5-6. Specifically, the irradiation intensity of the excimer lamp light551 a shown in FIG. 54 was regulated to set the heating temperature ofthe amorphous silicon thin film 522 at various temperatures in the rangeof from a room temperature to 1200° C., thereby polysilicon thin films523 were formed, and the crystal grain sizes and field effect mobilitieswere measured.

If the amorphous silicon thin film 522 is heated to about 300° C. orhigher, the crystal grain size of the polysilicon thin film 523increases along with the increase in the heating temperature as shown inFIG. 55, reaching the maximum of 5 μm at 1000° C. If the temperatureexceeds 1000° C., the surface of the glass substrate 521 partly fuses,hindering the crystal growth, and as a result, the crystal grain sizedecreases.

In addition, the field effect mobility of the polysilicon thin film 523increases along with the temperature increase as shown in FIG. 56, ifthe amorphous silicon thin film 522 is heated to about 300° C. orhigher, reaching the maximum of 450 cm²/V·sec at 1000° C. If thetemperature exceeds 1,000° C., the field effect mobility also decreases.

In other words, it is especially effective in increasing the crystalgrain size and improving the film quality of the polysilicon thin film523 that the glass substrate 521 is heated by the heater 561 in additionto the irradiation with the laser beam 571 a and at the same time theamorphous silicon thin film 522 is heated to the temperature in therange of 600 to 1100° C. by the irradiation with the excimer lamp light551 a.

In addition, a TFT was produced as in the foregoing Example 5-3 and thegate voltage (Vg)-drain current (Id) characteristic of the TFT thusprepared was measured. As shown in FIG. 50, which includes an example ofthe TFT in which the heating temperature of the amorphous thin film 522is 600° C., the present example exhibited even better TFTcharacteristics than the foregoing Examples 5-5 and 5-6.

EXAMPLE 5-8

Example 5-8 of the present invention will now be detailed with referenceto FIGS. 57 and 58, in addition to FIG. 50.

The present Example 5-8 differs from the foregoing Example 5-7 mainly inthat a pulsed Xe flash lamp 581 is used in place of the excimer lamp551.

Specifically, as shown in FIG. 57, the laser beam 571 a of the KrFexcimer laser 571 as in Example 5-7 is applied in such a manner that theregion 571 b to be irradiated in the amorphous silicon thin film 522 hasa belt-like shape of 500 μm×200 mm. On the other hand, the Xe flash lampbeam 581 a in the range from visible rays to ultraviolet rays, emittedfrom a flash Xe lamp 581, is applied to the region 581 b to beirradiated of 5 mm×200 mm including the region 571 b to be irradiatedwith the laser beam 571 a so that the amorphous silicon thin film 522 isheated to about 1000° C. The Xe flash lamp beam 581 a is, as shown inFIG. 58, synchronized with the irradiation pulse of the laser beam 571a, and applied so as to have a pulse width extending from before toafter the pulse width of the laser beam 571 a. The laser beam 571 a isalso so applied that the width of the irradiation pulse is not largerthan ⅔ of its irradiation cycle. Other conditions such as heatingconditions are the same as those in the foregoing Example 5-7.

The crystal grain size and field effect mobility of the polysilicon thinfilm 523 thus formed were substantially equal to those of the case wherethe amorphous silicon thin film 522 was heated to 1000° C. in theforegoing Example 5-7. Whereas in Example 5-7, some deformation wascaused in the glass substrate 521, no deformation was observed in thepresent Example 5-8, which facilitates the production of propersemiconductor circuits more reliably. Furthermore, the Xe flash lamp 581has a high heating efficiency and therefore can heat a large area perone time of the irradiation, which can readily improve the productivity.

In addition, a TFT was produced as in the foregoing Example 5-3 and thegate voltage (Vg)-drain current (Id) characteristic of the TFT thusprepared was measured. As shown in FIG. 50, the present exampleexhibited still better TFT characteristics than the foregoing Examples5-7.

EXAMPLE 5-9

Example 5-9 of the present invention will now be detailed with referenceto FIGS. 59 and 50.

In the present example 5-9, as shown in FIG. 59, the laser beam 571 aemitted from the same KrF excimer laser 571 as in Example 5-8 isreflected by the wavelength selective reflector 572 and applied to abelt-like shaped region 571 b to be irradiated having an area of about500 mm×200 mm in the amorphous silicon thin film 522. In addition, alaser beam 591 a from a YAG laser system 591, in which the laser beamemitted from a YAG laser is converted into a beam having a half of thewavelength by using a KTP crystal, is reflected by a reflector 592 andapplied to a region 591 b to be irradiated having an area of 5 mm×200 mmin the amorphous silicon thin film 522. As described above, the laserbeam 571 a and laser beam 591 a are reflected by the wavelengthselective reflector 572 and the reflector 592, and then enter theamorphous silicon thin film 522 perpendicularly. The conditions such asthe timing of irradiation with the laser beam 571 a and the laser beam591 a, the pulse width, the irradiation energy of the laser beam 571 a,and the heating temperature of the glass substrate 521 by the heater561, are the same as those in the foregoing Example 5-8.

The heating temperatures of the amorphous silicon thin film 522 by thelaser beam 591 a of the YAG laser system 591 were varied from roomtemperature to 1200° C. in the formation of the polysilicon thin films523, and the crystal grain sizes and field effect mobilities weremeasured. When the heating temperature of the amorphous silicon thinfilm 522 was 1100° C., both crystal grain size and field effect mobilityreached the maximums of 5.5 μm and 600 cm²/V·sec respectively. That is,because the YAG laser system 591 was used for preheating, the glasssubstrate 521 did not cause deformation or melt to cause impurities tomix into the polysilicon thin film 523, even if the amorphous siliconthin film 522 is heated to a relatively high temperature. Therefore,still better crystalline polysilicon thin film 523 was obtained thanthat of the foregoing Example 5-7 where the amorphous silicon thin film522 was heated by the excimer lamp 551. However, when the amorphoussilicon thin film 522 was further heated to 1200° C., both crystal grainsize and field effect mobility were decreased. The reason is thatmicrocrystalline silicon is already formed by the preheating with theYAG laser system 591, and this causes an adverse effect on the crystalgrowth process by the KrF excimer laser 571.

In addition, a TFT was produced as in the foregoing Example 5-3 and thegate voltage (Vg)-drain current (Id) characteristic of the TFT thusprepared was measured. As shown in FIG. 50, the present exampleexhibited still better TFT characteristics than the foregoing Examples5-8.

The laser system for preheating is not limited to the YAG laser system591 described above. The same advantageous effects can be achieved byusing, for example, a pulsed laser such as an XeCl excimer laser if theirradiation is implemented with a wavelength different from thewavelength of the KrF excimer laser 571 and with a pulse width longerthan the KrF excimer laser 571 depending on the mixing ratio of the gas,and further with the timing shown in FIG. 58. In addition, acontinuous-wave laser system such as an Ar laser or the like may also beemployed.

The foregoing examples describe the examples in which silicon (Si) isemployed as a semiconductor, but the invention is not so limited. It hasbeen confirmed that the same advantageous effects can be obtained byIII-V compound semiconductors such as germanium (Ge) and galliumarsenide (GaAs), and II-VI compound semiconductors such as zinc selenide(ZnSe). Further possible examples include silicon carbide (SiC) andsilicon germanium (SiGe).

It is also noted that the irradiation of the amorphous silicon thin film522 with the laser beam 531 a may be performed in such a manner that thelaser beam 531 a is applied from the direction of the glass substrate521, or from both directions of the amorphous silicon thin film 522 andthe glass substrate 521.

In addition, in place of the glass substrate 521, other substrates maybe used, and the examples of usable substrates include a substrate madeof quartz or organic materials such as plastics and a substrate havingan insulating film formed on a surface of a conductive substrate.

Further, the laser beam 532 a or the like for preheating may be appliedin such a manner that the laser beam 532 a or the like is applied not tothe entire region of the amorphous silicon thin film 522 but only to aregion where high TFT characteristics are required, while the otherregion is, as in the prior art, irradiated only with the laser beam 531a or the like for crystallization.

EXAMPLE 6-1

An example in which a thin film transistor as a semiconductor device isemployed for a liquid crystal display device will be explained in thefollowing.

In an active matrix-type liquid crystal display device, thin filmtransistors in the image display region are required to have a highuniformity in transistor characteristics in order to reduce unevennessin the displayed images, whereas thin film transistors for the drivingcircuit disposed in a peripheral region adjacent to the image displayregion are required to have a high response characteristic. However,despite the extensive research on various methods for growing crystals,it is not easy to achieve both the uniformity in characteristics and thehigh response characteristic at the same time. In view of this, in thepresent example, a plurality of regions are provided in a semiconductorfilm (amorphous silicon layer) and the regions are irradiated indifferent manners of irradiation, thereby making it possible that eachregion obtains required characteristics. Specifically, a first laserlight is applied to the entire surface or to only the image displayregion, and thereafter, a second laser light is applied, with the secondlaser light having a higher energy density than the first laser light.With reference to the drawings, a laser annealing apparatus and a methodof laser annealing are specifically detailed below.

A laser annealing apparatus employed in the present example may have aconfiguration similar to the prior art apparatus shown in FIG. 9. InFIG. 9, there are shown a laser oscillator 151, a reflector 152, ahomogenizer 153, a window 154, a substrate 155 having an amorphoussilicon layer formed thereon, a stage 156, and a control unit 157.

In the laser annealing of the amorphous silicon layer, the laser lightemitted from the laser oscillator 151 is led to the homogenizer 153 bythe reflector 152, so as to be shaped in a predetermined shape with auniform energy, and the shaped laser beam is applied through the window154 to the substrate 155 mounted on the stage 156 in the treatmentchamber. In the present example, however, the control unit 157 is soconfigured that the laser light can be applied exclusively each of aplurality of predetermined regions in the substrate 155 and theirradiation conditions can be varied for each region to be irradiated.

Using the laser annealing apparatus described above, a first laser lightirradiation is performed in the following manner; a laser light shapedby using the homogenizer 153 into a line-like cross-sectional shape (forexample, 300 μm wide and 10 cm long) is applied to the entire region ofthe substrate 155 so that the energy density becomes 280 mJ/cm², whilethe substrate 155 is being moved so that the regions to be irradiatedare partially overlapped each other (scan irradiation using a line-likeshaped laser light). This laser light irradiation may be performedexclusively in the image display region 155 a in FIG. 60.

Next, a second laser light irradiation is performed in the followingmanner; a laser light is applied to the driving circuit regions 155 band 155 c with an energy density of 400 mJ/cm², which is higher thanthat in the first laser light irradiation (scan irradiation using aline-like shaped laser light).

The substrate 155 is, for example, such a substrate that an amorphoussilicon layer is formed on a glass substrate by plasma CVD to have athickness of 500 Å, and thereafter the substrate with the amorphoussilicon layer is dehydrogenation-treated for one hour at 450° C. Thelaser light is, for example, such that a laser light oscillates with apulse width of 25 ns and an interval of 300 Hz. The laser light scansthe substrate 155 in a relative manner by moving the substrate 155 at apredetermined speed. In the second laser light irradiation, the scan isperformed so that, as shown in FIG. 61, the regions to be irradiatedoverlap by 30 μm in each of the regions (the rate of overlap being 10%).In this case, uneveness is caused in characteristics such as mobilitiesbetween an overlapped region in which the laser light is repeatedlyapplied and the rest of the regions. However, as shown in the figure,such uneveness in TFT characteristics and so forth can be readilyminimized by forming the TFT 610 or the like so as to avoid theoverlapped region, and using the overlapped region for wiring pattern orthe like. Further, in the second laser light irradiation, a total timefor the irradiation can be reduced if the direction of line-like shapedbeam is made parallel to each of the sides of the substrate 155 (thedirections designated by the solid lines in the driving circuit regions155 b and 155 c) and the scanning is performed in the directionsperpendicular to each of the sides. To make this possible, the stage onwhich the substrate 155 is mounted is rotated 90 degrees in the laserirradiation. (It is possible to rotate 90 degrees the direction of theline-like shaped beam, but generally, this is difficult.)

By the first laser light irradiation described above, crystallization isperformed in such a manner as to ensure the uniformity of semiconductorfilm characteristics required for the image display region 155 a, whileby the second layer light irradiation, high field effect mobilities areachieved in the driving circuit regions 155 b and 155 c. Specifically,while performing laser light irradiations under various irradiationconditions, the present inventors found that in a scan irradiation withan energy density of 300 mJ/cm² or greater, uneveness in field effectmobilities tends to occur in the overlapped region in each scan. In viewof this problem, the laser light is applied with an energy density lowerthan 300 mJ/cm² to the image display region 155 a, in which uniformityof characteristics of polycrystal silicon in a plane is required,whereas the laser light is applied with an energy density higher than300 mJ/cm² to the driving circuit regions 155 b and 155 c, in which goodcharacteristics such as a high field effect mobility are required.Thereby, without achieving both uniformity and improvement of filmcharacteristics at the same time, polycrystal silicon layers havingdifferent characteristics from each other, each layers in which adifferent requirement is met, can be formed respectively in the imagedisplay region 155 a and the driving circuit regions 155 b and 155 c.

EXAMPLE 6-2

Another example in which a thin film transistor is employed for a liquidcrystal display device will be explained in the following.

The present example differs from the foregoing Example 6-1 in that thesecond laser light irradiation employs a laser beam having a square-likecross-sectional shape, whereas the first laser light irradiation employsa laser beam having a line-line cross-sectional shape as in theforegoing Example 6-1.

The laser annealing apparatus in the present example differs from theapparatus in FIG. 9 in that, as shown in FIGS. 62(a) and 62(b), ahomogenizer A 621 for shaping the cross-sectional beam shape of a laserlight into a line-like shape and a homogenizer B 622 for shaping thecross-sectional beam shape into a square-like shape (1 cm square, forexample) are employed in place of the homogenizer 153. (Like orcorresponding parts shown in FIG. 9 are designated by like referencenumerals or characters in FIGS. 62(a) and 62(b) and will not be furtherelaborated upon.)

Using the laser annealing apparatus described above, firstly as shown inFIG. 62(a), a laser light having a line-like cross-sectional shape isapplied via the homogenizer A 621 to the entire region of the substrate155 or only the image display region 155 a with an energy density of 280mJ/cm², at which energy density the uniformity is ensured (scanirradiation using a line-like shaped laser light). Thereafter, using thehomogenizer B 622 as shown in FIG. 62(b), a laser light having asquare-like cross-sectional shape is applied, as shown in FIG. 63, toeach of the irradiated regions 631 and 632 with an energy density of 400mJ/cm² (scan irradiation using a square-like shaped laser light).

By employing a laser light having a square-like cross-sectional beamshape as above, it is made possible to laser-anneal the driving circuitregions 155 b and 155 c without rotating the substrate 155 by 90 degreesas in the case of the foregoing Example 6-1. Accordingly, in the presentexample, polycrystal silicons having different characteristics in thepixel region and the driving circuit region can be obtained as well asin the foregoing Example 6-1, and in addition, the simplification in theapparatus and the fabrication steps can be readily achieved.

EXAMPLE 6-3

The second laser light irradiation in the foregoing Example 6-2 may beperformed a plurality of times. Specifically, in the second laser lightirradiation as in the foregoing Example 6-2, the substrate 155 is notmoved, and by fixing the regions to be irradiated with the laser light,the laser irradiation may be performed in a stationary sate for each ofthe regions 631 and 632 corresponding to the square-like laser beamshape. When each of the regions 631, 632 to be irradiated is overlappedby, for example, about 30 μm in the case of the laser light having 1 cmsquare cross-sectional shape, the field effect mobility in the regionwhere the laser light is not overlapped can be readily increased to aremarkable degree, and the uniformity in the region can also beimproved. Here, if the energy density of the laser light is high asdescribed above, uneveness is caused in characteristics such asmobilities between an overlapped region in which the laser light isrepeatedly applied and the rest of the regions. However, for the drivingcircuit regions 155 b and 155 c, the uniformity throughout the entireregion as required for the image display region 155 a is not necessarilyrequired. In view of this, the driving circuit should be formed so thatthe pattern of the semiconductor film (TFT pattern) avoids theoverlapped region of the laser shot (the edge of the laser beam), andthe overlapped region should be used for wiring pattern or the like. Inother words, only the portion of the polysilicon in whichcharacteristics are uniform should be used for forming TFTs or the like.If the overlapped regions are not used as such, the area of theoverlapped regions is relatively small, and therefore the utilizationefficiency of the driving circuit regions 155 b and 155 c is not sodegraded.

In addition, the film characteristics can be further improved byperforming the laser light irradiation a plurality of times (for example30 times) for each of the regions 631 and 632FIG. 64 illustrates therelationship between the number of the irradiation in the stationarystate and the mobility of the resulting polycrystal silicon. As apparentfrom FIG. 64, the number of the laser irradiation has a preferablerange, and the mobility decreases outside the preferable range. In thecase of the energy density in the stationary irradiation being 400mJ/cm², the number of the irradiation is preferable to be 50 times ormore, or more preferably in -the range of 80 times to 400 times, toobtain a polycrystal silicon having a high field effect mobility. It isnoted that the effect of improving the field effect mobility byperforming the laser light irradiation in a stationary state can be alsoobtained in the case of employing a line-like shaped laser light,although the effect is not as great as that in the case of the squarelike shaped laser light.

EXAMPLE 6-4

The conditions of the laser light irradiation for the image displayregion 155 a are differentiated from that for the driver circuit regions155 b, 155 c in the foregoing Examples 6-1 to 6-3. Additionally, theconditions of the laser light irradiation may be further varied bydividing the region to be irradiated into further more regions in thedriving circuit region, in order to obtain polycrystal silicons havingdifferent characteristics within the driving circuit region.Specifically, for example, in the second laser light irradiation in theforegoing Example 6-3, a region in the driving circuit regions 155 b and155 c in which a transfer gate in the latch or shift resistor is formedmay be irradiated with a high energy density (for example 400 mJ/cm²)since such a region requires a high mobility, and the other region maybe irradiated with an energy density of about 330 mJ/cm², in order toachieve, rather than the mobility, the uniformity obtained by thereduction in noises and adjustment variations and the improvement inproductivity attained by making the region to be irradiated large. Thevariations in the conditions of the irradiation can be also attained byvarying the number of times of the irradiation, which will results insubstantially the same effect.

EXAMPLE 6-5

Still another example of the laser annealing apparatus will bedescribed.

The laser annealing apparatus in this example differs from the apparatusin FIG. 9 in that a mask 641 in which a transmissivity of a laser lightis partially different is provided between the window 154 and the stage156, as shown in FIG. 65. The mask 641 comprises an attenuating region641 a corresponding to the image display region 155 a of the substrate155 and a transmitting region 641 b corresponding to the driver circuitregions 155 b, 155 c, as shown in FIG. 66. Specifically, for example,transmissivities of the laser light can be partially determined at adesired value by a quartz plate being partially covered with such anoptical thin film as an ND filter or a dielectric multilayer film, andthereby the energy density of the laser light irradiation can be reducedin the pixel region.

By employing the laser annealing apparatus described above, the laserlight shaped into a line-like cross-sectional shape is applied to theentire region in the substrate with an energy density of 400 mJ/cm²while the laser beam or the mask 641 and the stage 154 is/are beingmoved. This makes it possible for the image display region 155 a to belaser-annealed with an energy density of 280 mJ/cm², as in the foregoingExample 6-1. That is, it is made possible to form semiconductor filmshaving different characteristics between a pixel region and a drivingcircuit region at one time.

Although the mask 641 is disposed so that a certain distance is providedbetween the mask 641 and the window 154 and between the mask 641 and thesubstrate 155 as shown in FIG. 65, but the present example is not solimited. The mask 641 may be disposed so as to be in contact with thesubstrate 155 to improve the flatness of the surface to be irradiated,or may be integrally formed with the window 154. Further, the mask 641may be provided in the homogenizer 153, or may be not such a mask thatthe intensity of laser light is attenuated, but such that the intensityof the laser light is varied by a refracting optical system or the like.

EXAMPLE 6-6

An example of a laser annealing apparatus that can further improve theuniformity in the image display region will be given below.

In the laser annealing apparatus of Example 6-6, a homogenizing opticalelement 651 for scattering an incoming laser beam is provided over thesubstrate 155, as shown in FIG. 67. Thereby, it is made possible toreduce unevenness in the luminous energy caused by such as diffraction,the shape of the laser beam, and the like. It is also made possible toprevent instability of the laser pulse resulting from the reflectedlight from the substrate 155 returning to the laser oscillator.

In addition, as shown in FIG. 68, a combined homogenizing opticalelement 652 may be employed to simultaneously form a region having ahigh uniformity and a region having a high crystallinity in thesemiconductor layer. The combined homogenizing optical element 652 has ascattering region 652 a and a transmission region 652 b having a mirrorfinish or the like.

It is to be understood that while the respective examples describedabove can achieve the respective advantageous effects as describedabove, the foregoing examples may be combined with each other to obtainsynergistic effects in addition to the respective effects, insofar asthe achieved effects are consistent with each other.

INDUSTRIAL APPLICABILITY

As has been described thus far, according to the present invention, thefollowing advantageous effects are achieved.

Firstly, according to the present invention, it is possible to produce apolycrystal silicon thin film in which the region where a transistor isto be formed has a larger grain size, thereby to remarkably improvetransistor characteristics such as the field effect mobility, and alarge scale driving circuit can be integrated for example in a liquidcrystal display device and so forth. In addition, the hydrogen contentand the stress in the thin film can be reduced by employing a siliconoxynitride thin film in which oxygen is added to silicon nitride as aninsulating film, and thereby, a more stable transistor can be obtained.Furthermore, the grain size and orientation of crystals can becontrolled, and the interference between the crystals in the process ofgrowth can be prevented, which leads to a sufficient grain size.Furthermore, in the present invention, crystal nuclei starts to beformed in the peripheral regions earlier than in prior art, and as aresult, it is possible for crystal growth to proceed quicker than inprior art.

In addition, according to the present invention, at least the channelregion on the non-single crystalline semiconductor layer is providedwith a region for controlling the orientation of crystal growth such asa gap for controlling an orientation of crystal growth or the like,which controls the crystal growth in the directions of the source regionand the drain region. Thus, large crystal grains longitudinallyextending in the direction linking the source region and the drainregion are formed, and thereby a crystalline thin film transistor havinga small density of crystal boundaries is attained, the transistor beingexcellent in TFT characteristics such as the field effect mobility.

According to the present invention, furthermore, the uniformity andcrystallinity of crystal grains are further improved by a means ofappropriately adjusting an intensity pattern of the light beam. Thus, acrystallized region having a high degree of field effect mobility can beformed on a limited or specific region on the substrate withoutproducing adverse effects on other circuits. As a consequence, it ismade possible to integrally form pixel transistors and a drivingcircuit, which requires several tens of times to several hundreds oftimes higher mobility, on the same substrate. It is also made possibleto integrally form CPUs and the like on a single substrate, andtherefore, according to the invention, high-performance AM-LCDs having ahigh degree of integration can be produced at low cost.

In addition, by applying at least two types of energy beams resulting indifferent absorption indices of a precursor semiconductor film, theprecursor semiconductor film is heated throughout the thicknessdirection and the substrate is also heated. As a result, the precursorsemiconductor film is crystallized while being annealed. Thereby crystalgrowth is promoted, enabling relatively large crystal grains, andfurther, crystal defects are reduced, improving electricalcharacteristics of the semiconductor film. Moreover, in comparison withthe case of using a heater or the like, the time for heating thesubstrate can be reduced, which leads to an improvement in theproductivity.

In addition, it is possible to form a plurality of regions havingdifferent characteristics, one region having good semiconductor filmcharacteristics and another having a high uniformity. Thereby, both thegood characteristics necessary for the circuit region and the highuniformity required for the pixel region are achieved in, for example, athin film transistor array for use in a liquid crystal panel having abuilt-in peripheral driving circuits.

What is claimed is:
 1. A method of producing a semiconductor thin film,comprising the steps of: forming a non-single crystal semiconductor thinfilm having a protruding part extending outwardly in the same plane as aplane of said non-single crystal semiconductor thin film; and growingcrystals in said non-single crystal semiconductor thin film byirradiating with a pulse-like energy beam.
 2. A method of producing asemiconductor thin film according to claim 1, wherein said energy beamincludes at least one of a laser light, an electron beam, and an ionbeam.
 3. A method of producing a semiconductor thin film according toclaim 2, wherein said energy beam includes an excimer laser light.
 4. Amethod of producing a semiconductor thin film wherein a crystallizationof a non-single crystal semiconductor thin film is effected by anannealing treatment by irradiating said non-single crystal semiconductorthin film with an energy beam, wherein: said irradiation with the energybeam is such that said irradiation is completed substantially at onetime at least in a predetermined region; and crystal nuclei in aperipheral region in said non-single crystal semiconductor thin film areformed earlier than crystal nuclei in a central region in saidnon-single crystal semiconductor thin film, and thereafter, said crystalnuclei in said peripheral region are grown towards said central regionbefore the crystal nuclei in said central region start to be formed orgrown.
 5. A method of producing a semiconductor thin film according toclaim 4, wherein, in a semiconductor thin film anneal-treated, saidcrystal nuclei in said peripheral region are formed earlier than saidcrystal nuclei in said central region by cooling said peripheral regionearlier than said central region.
 6. A method of producing asemiconductor thin film according to claim 5, wherein said peripheralregion comprises a peripheral edge region having a substantiallyprotruding shape, a plurality of directions for a heat generated andaccumulated by an annealing treatment in said peripheral edge region toescape in a plane parallel to said semiconductor thin film are providedso that said peripheral region is cooled earlier than said centralregion.
 7. A method of producing a semiconductor device including acrystalline semiconductor layer, said crystalline semiconductor layercomprising a channel region, a source region disposed at both sides ofsaid channel region, and a drain region, said method comprising:depositing a non-single crystalline thin film on an insulatingsubstrate; forming an early-crystallization region by ion-implanting animpurity in a partial region in said non-single crystallinesemiconductor thin film, said impurity for raising acrystallization-starting temperature of said partial region, saidearly-crystallization region having a belt-like shape longitudinallyextending in a direction linking said source region and said drainregion; and after said step of forming an early-crystallization region,irradiating said thin film with an energy beam to crystallize said thinfilm.
 8. A method of producing a semiconductor device including acrystalline semiconductor layer, said crystalline semiconductor layercomprising a channel region, a source region disposed at both sides ofsaid channel region, and a drain region, said method comprising:depositing a non-single crystalline thin film on an insulatingsubstrate; forming an early-crystallization region by ion-implanting animpurity in a partial region in said non-single crystallinesemiconductor thin film, said early-crystallization region being dividedinto a plurality of early-crystallization regions discontinuouslydisposed in a direction linking said source region and said drainregion, said impurity for raising a crystallization-starting temperatureof said partial region; and after said step of forming anearly-crystallization region, irradiating said thin film with an energybeam to crystallize said thin film.
 9. A method of producing asemiconductor device including a crystalline semiconductor layer, saidcrystalline semiconductor layer comprising a channel region, a sourceregion disposed at both sides of said channel region, and a drainregion, said method comprising: depositing a non-single crystalline thinfilm on an insulating substrate; forming an early-crystallization regionby ion-implanting an impurity in a partial region in said non-singlecrystalline semiconductor thin film, said impurity for raising acrystallization-starting temperature of said partial region, and aftersaid step of forming an early-crystallization region, irradiating saidthin film with an energy beam to crystallize said thin film, said energybeam being an excimer laser beam.
 10. A method of producing asemiconductor thin film wherein a thin film comprising a non-singlecrystalline material formed on a substrate is irradiated with a lightbeam and thereafter cooled whereby said non-single crystalline materialis crystallized or recrystallized, said method including: using anatmosphere gas that is hydrogen gas, and maintaining a pressure of saidatmosphere gas at 10⁻⁵ torr or higher to cause an uneven temperaturedistribution on a surface of said thin film irradiated with said lightbeam.
 11. A method of producing a semiconductor film wherein a precursorsemiconductor film formed on a substrate is irradiated with a firstenergy beam supplying said precursor semiconductor film with at leastsuch an energy that said precursor semiconductor film can becrystallized, and with a second energy beam such that an absorptionindex of said precursor semiconductor film is smaller than an absorptionindex by said first energy beam and an energy supplied by said secondenergy beam is smaller than an energy capable of crystallizing saidprecursor semiconductor film thereby heating said substrate, and a timeand an area of irradiating with said first energy beam is within a timeand an area of irradiating with said second energy beam.
 12. A method ofproducing a semiconductor film according to claim 11, wherein saidprecursor semiconductor film is an amorphous silicon thin film.
 13. Amethod of producing a semiconductor film according to claim 11, wherein:said first energy beam is such that an absorption coefficient of saidprecursor semiconductor film is approximately equal to or greater thanthe reciprocal of a film thickness of said precursor semiconductor film;and said second energy beam is such that an absorption coefficient ofsaid precursor semiconductor film is approximately equal to or less thanthe reciprocal of a film thickness of said precursor semiconductor film.14. A method of producing a semiconductor film according to claim 11,wherein: said first energy beam is such that an absorption coefficientof said precursor semiconductor film is approximately 10 times orgreater than the reciprocal of a film thickness of said precursorsemiconductor film; and said second energy beam is such that anabsorption coefficient of said precursor semiconductor film isapproximately the reciprocal of a film thickness of said precursorsemiconductor film.
 15. A method of producing a semiconductor filmaccording to claim 11, wherein said first and second energy beams have adifferent wavelength from each other.
 16. A method of producing asemiconductor film according to claim 15, wherein: said first energybeam is an energy beam having a single wavelength; and said secondenergy beam includes at least a wavelength component in a visible lightrange.
 17. A method of producing a semiconductor film according to claim16, wherein: said first energy beam is a laser light; and said secondenergy beam is an infrared lamp.
 18. A method of producing asemiconductor film according to claim 16, wherein: said first energybeam is a laser light; and said second energy beam is an incandescentlight.
 19. A method of producing a semiconductor film according to claim16, wherein: said first energy beam is a laser light; and said secondenergy beam is an excimer lamp light.
 20. A method of producing asemiconductor film according to claim 15, wherein said second energybeam contains at least a wavelength component from a visible light rangeto an ultraviolet range.
 21. A method of producing a semiconductor filmaccording to claim 20, wherein: said first energy beam is a laser light;and said second energy beam is a xenon flash lamp light.
 22. A method ofproducing a semiconductor film according to claim 15, wherein said firstenergy beam and said second energy beam are a laser light.
 23. A methodof producing a semiconductor film according to claim 22, wherein: saidprecursor semiconductor film is an amorphous silicon thin film; saidfirst energy beam is one laser light selected from an argon fluorideexcimer laser, a krypton fluoride excimer laser, a xenon chlorideexcimer laser, and a xenon fluoride excimer laser; and said secondenergy beam is a laser light of an argon laser.
 24. A method ofproducing a semiconductor film according to claim 22, wherein: saidsubstrate is a glass substrate; said precursor semiconductor film is anamorphous silicon thin film; said first energy beam is one laser lightselected from an argon fluoride excimer laser, a krypton fluorideexcimer laser, a xenon chloride excimer laser, and a xenon fluorideexcimer laser; and said second energy beam is a laser light of a carbondioxide gas laser.
 25. A method of producing a semiconductor filmaccording to claim 11, wherein said first energy beam and said secondenergy beam are applied to a belt-like shaped region in said precursorsemiconductor film.
 26. A method of producing a semiconductor filmaccording to claim 11, wherein a region in said precursor semiconductorfilm to be irradiated with said second energy beam is larger than aregion in said precursor semiconductor film to be irradiated with saidfirst energy beam, and includes said region to be irradiated with saidfirst energy beam.
 27. A method of producing a semiconductor filmaccording to claim 11, wherein said first energy beam and said secondenergy beam are incident approximately perpendicular to said precursorsemiconductor film.
 28. A method of producing a semiconductor filmaccording to claim 11, wherein said second energy beam is applied atleast prior to applying said first energy beam.
 29. A method ofproducing a semiconductor film according to claim 28, wherein saidsubstrate on which said precursor semiconductor film is formed is moved,and said second energy beam is applied to a more forward position insaid precursor semiconductor film with respect to a direction of movingof said substrate than a position where said first energy beam isapplied.
 30. A method of producing a semiconductor film according toclaim 11, wherein: said first energy beam is intermittently applied; andsaid second energy beam is continuously applied.
 31. A method ofproducing a semiconductor film according to claim 30, wherein: saidfirst energy beam is a pulsed laser light; and said second energy beamis a continuous-wave laser light.
 32. A method of producing asemiconductor film according to claim 30, wherein: said first energybeam is a pulsed laser light; and said second energy beam is a lamplight.
 33. A method of producing a semiconductor film according to claim11, wherein said first energy beam and said second energy beam aresynchronized with each other and intermittently applied.
 34. A method ofproducing a semiconductor film according to claim 33, wherein a time ofirradiating with said first energy beam is within a time of irradiatingwith said second energy beam, and is two-thirds or shorter of anirradiation cycle of said second energy beam.
 35. A method of producinga semiconductor film according to claim 33, wherein said first energybeam and said second energy beam are a pulsed laser light.
 36. A methodof producing a semiconductor film according to claim 33, wherein: saidfirst energy beam is a pulsed laser light; and said second energy beamis an intermittently-operated lamp light.
 37. A method of producing asemiconductor film according to claim 11, wherein said first energy beamand said second energy beam are applied so that said precursorsemiconductor film is heated at a temperature of 300° C. to 1200° C. 38.A method of producing a semiconductor film according to claim 11,wherein said first energy beam and said second energy beam are appliedso that said precursor semiconductor film is heated at a temperature of600° C. to 1100° C.
 39. A method of producing a semiconductor filmaccording to claim 11, further comprising a step of heating saidsubstrate on which said precursor semiconductor film is formed with aheater.
 40. A method of producing a semiconductor film according toclaim 39, wherein said substrate on which said precursor semiconductorfilm is formed is heated at a temperature of 300° C. to 600° C.
 41. Amethod of producing a semiconductor film according to claim 11, wherein:said first energy beam is applied to a plurality of regions in saidprecursor semiconductor film; and said second energy beam is applied toonly a part of the plurality of regions.
 42. A method of producing asemiconductor film according to claim 11, wherein said second energybeam is such that an absorption index of said substrate is larger thanan absorption index of said precursor semiconductor film.
 43. A methodof producing a semiconductor film according to claim 42, wherein saidfirst energy beam is such that an absorption coefficient of saidprecursor semiconductor film is approximately 10 times or greater thanthe reciprocal of a film thickness of said precursor semiconductor film.44. A method of producing a semiconductor film according to claim 42,wherein: said substrate is a glass substrate; said precursorsemiconductor film is an amorphous silicon thin film; said first energybeam is one laser light selected from an argon fluoride excimer laser, akrypton fluoride excimer laser, a xenon chloride excimer laser, and axenon fluoride excimer laser; and said second energy beam is a laserlight of a carbon dioxide gas laser.
 45. A method of producing asemiconductor thin film comprising a step of irradiating a non-singlecrystal semiconductor thin film with an energy beam, said non-singlecrystal semiconductor thin film formed on a substrate having an imagedisplay region and a driving circuit region, said method characterizedin that: a first irradiation of said image display region is a scanningirradiation such that said substrate is scanned by said energy beam in arelative manner and a region to be irradiated with said energy beam isshifted with a predetermined overlap; and a second irradiation of saiddriving circuit region is a stationary irradiation with a higher energydensity than said first irradiation such that said energy beam is fixedwith respect to said substrate in a relative manner.
 46. A method ofproducing a semiconductor thin film according to claim 45, wherein saidsecond irradiation is performed a plurality of times in a state wheresaid energy beam is fixed with respect to said substrate in a relativemanner.
 47. A method of producing a semiconductor thin film comprising astep of irradiating a non-single crystal semiconductor think film withan energy beam, said non-single crystal semiconductor thin film formedon a substrate having an image display region and a driving circuitregion, said method characterized in that: said image display region anda plurality of predetermined regions in said driving circuit region areirradiated with said energy beam at different energy densities from eachother, and said image display region is irradiated with said energy beamat a higher energy density than said image display region; and in saidplurality of regions in said driving circuit region, a region in which atransfer gate constituting one of a latch circuit and a shift registeris formed is irradiated with said energy beam at a higher energy densitythan other regions.
 48. A method of producing a semiconductor thin filmcomprising a step of irradiating a non-single crystal semiconductor thinfilm with an energy beam, said non-single crystal semiconductor thinfilm formed on a substrate having an image display region and a drivingcircuit region, said method including: applying said energy beam to anentire region of said substrate through a filter in which a region ofsaid filter corresponding to said image display region has a lowertransmissivity than a region of said filter corresponding to saiddriving circuit region, and said image display region and said drivingcircuit region are simultaneously irradiated with said energy beam. 49.A method of producing a semiconductor thin film comprising a step ofirradiating a non-single crystal semiconductor thin film with an energybeam, said non-single crystal semiconductor thin film formed on asubstrate, said method characterized in that: said energy beam isapplied via a homogenizing element having a property of scattering saidenergy beam.
 50. A method of producing a semiconductor thin filmaccording to claim 49, wherein: said homogenizing element has a partialregion having a property of transmitting said energy beam; and saidenergy beam incident on said partial region having a property oftransmitting is allowed to transmit through said partial region toirradiate said non-single crystal semiconductor thin film.